diff --git a/src/rules/requests/request-rule.ts b/src/rules/requests/request-rule.ts
index e32c8ad7a..ae0d5dde2 100644
--- a/src/rules/requests/request-rule.ts
+++ b/src/rules/requests/request-rule.ts
@@ -79,6 +79,7 @@ export class RequestRule implements RequestRule {
 
     handle(req: OngoingRequest, res: OngoingResponse, options: {
         record?: boolean,
+        bufferRequestBody?: boolean,
         debug: boolean,
         keyLogStream?: Writable,
         emitEventCallback?: (type: string, event: unknown) => void
@@ -88,6 +89,7 @@ export class RequestRule implements RequestRule {
                 const result = await step.handle(req, res, {
                     emitEventCallback: options.emitEventCallback,
                     keyLogStream: options.keyLogStream,
+                    bufferRequestBody: options.record || options.bufferRequestBody,
                     debug: options.debug
                 });
 
diff --git a/src/rules/requests/request-step-impls.ts b/src/rules/requests/request-step-impls.ts
index 4ac122875..c59f9d49d 100644
--- a/src/rules/requests/request-step-impls.ts
+++ b/src/rules/requests/request-step-impls.ts
@@ -29,6 +29,7 @@ import { AbortError } from '../../util/abort-error';
 import { isAbsoluteUrl, getEffectivePort } from '../../util/url';
 import {
     waitForCompletedRequest,
+    streamBodyWithoutBuffering,
     buildBodyReader,
     isHttp2,
     writeHead,
@@ -164,6 +165,7 @@ export interface RequestStepOptions {
     emitEventCallback?: (type: string, event: unknown) => void;
     keyLogStream?: Writable;
     debug: boolean;
+    bufferRequestBody?: boolean;
 }
 
 export class FixedResponseStepImpl extends FixedResponseStep {
@@ -484,9 +486,19 @@ export class PassThroughStepImpl extends PassThroughStep {
             `);
         }
 
-        // We have to capture the request stream immediately, to make sure nothing is lost if it
-        // goes past its max length (truncating the data) before we start sending upstream.
-        const clientReqBody = clientReq.body.asStream();
+        const needsRequestBody = !!(
+            options.bufferRequestBody ||
+            this.beforeRequest ||
+            this.beforeResponse ||
+            this.transformRequest?.updateJsonBody ||
+            this.transformRequest?.patchJsonBody ||
+            this.transformRequest?.matchReplaceBody
+        );
+        // Capture replay streams before async setup so truncation cannot discard
+        // bytes before forwarding starts. Otherwise retain normal stream backpressure.
+        const clientReqBody = needsRequestBody
+            ? clientReq.body.asStream()
+            : streamBodyWithoutBuffering(clientReq.body);
 
         const isH2Downstream = isHttp2(clientReq);
 
diff --git a/src/server/mockttp-server.ts b/src/server/mockttp-server.ts
index 693d82212..086397178 100644
--- a/src/server/mockttp-server.ts
+++ b/src/server/mockttp-server.ts
@@ -768,6 +768,8 @@ export class MockttpServer extends AbstractMockttp implements Mockttp {
                 if (this.debug) console.log(`Request matched rule: ${nextRule.explain()}`);
                 await nextRule.handle(request, response, {
                     record: this.recordTraffic,
+                    bufferRequestBody: emitter.listenerCount('request') > 0 ||
+                        emitter.listenerCount('request-body-data') > 0,
                     debug: this.debug,
                     keyLogStream: this.keyLogStream,
                     emitEventCallback: (emitter.listenerCount('rule-event') !== 0)
diff --git a/src/util/request-utils.ts b/src/util/request-utils.ts
index ba3b9b328..dd9ee3318 100644
--- a/src/util/request-utils.ts
+++ b/src/util/request-utils.ts
@@ -142,11 +142,24 @@ export async function decodeBodyBuffer(buffer: Buffer, headers: Headers) {
     )
 }
 
+const unbufferedBodyReaders = new WeakMap<OngoingBody, () => stream.Readable>();
+
 // Parse an in-progress request or response stream, i.e. where the body or possibly even the headers have
 // not been fully received/sent yet.
 const parseBodyStream = (bodyStream: stream.Readable, maxSize: number, getHeaders: () => Headers): OngoingBody => {
     let bufferPromise: BufferInProgress | null = null;
     let completedBuffer: Buffer | null = null;
+    let streamTaken = false;
+
+    const capture = (): BufferInProgress => {
+        if (!bufferPromise) {
+            bufferPromise = streamToBuffer(bodyStream, maxSize);
+            bufferPromise
+                .then((buffer) => completedBuffer = buffer)
+                .catch(() => {}); // If we get no body, completedBuffer stays null
+        }
+        return bufferPromise;
+    };
 
     let body = {
         // Returns a stream for the full body, not the live streaming body.
@@ -154,24 +167,19 @@ const parseBodyStream = (bodyStream: stream.Readable, maxSize: number, getHeader
         // and buffered data, and then continues with the live stream, if active.
         // Listeners to this stream *must* be attached synchronously after this call.
         asStream() {
+            if (streamTaken) throw new Error('Cannot replay an unbuffered body stream');
             // If we've already buffered the whole body, just stream it out:
             if (completedBuffer) return bufferToStream(completedBuffer);
 
             // Otherwise, we want to start buffering now, and wrap that with
             // a stream that can live-stream the buffered data on demand:
-            const buffer = body.asBuffer();
+            const buffer = capture();
             buffer.catch(() => {}); // Errors will be handled via the stream, so silence unhandled rejections here.
             return bufferThenStream(buffer, bodyStream);
         },
         asBuffer() {
-            if (!bufferPromise) {
-                bufferPromise = streamToBuffer(bodyStream, maxSize);
-
-                bufferPromise
-                    .then((buffer) => completedBuffer = buffer)
-                    .catch(() => {}); // If we get no body, completedBuffer stays null
-            }
-            return bufferPromise;
+            if (streamTaken) return Promise.reject(new Error('Cannot replay an unbuffered body stream'));
+            return capture();
         },
         async asDecodedBuffer() {
             const buffer = await body.asBuffer();
@@ -188,9 +196,24 @@ const parseBodyStream = (bodyStream: stream.Readable, maxSize: number, getHeader
         },
     };
 
+    unbufferedBodyReaders.set(body, () => {
+        // Matchers or earlier steps may already have read some of the body.
+        // Reuse their replay stream instead of losing that prefix.
+        if (bufferPromise) return body.asStream();
+        if (streamTaken) throw new Error('Cannot replay an unbuffered body stream');
+        streamTaken = true;
+        return bodyStream;
+    });
+
     return body;
 }
 
+/** @internal Consume a body once unless an existing reader requires replay. */
+export function streamBodyWithoutBuffering(body: OngoingBody): stream.Readable {
+    const read = unbufferedBodyReaders.get(body);
+    return read ? read() : body.asStream();
+}
+
 async function runAsyncOrUndefined<R>(func: () => Promise<R>): Promise<R | undefined> {
     try {
         return await func();
diff --git a/test/integration/proxying/request-buffering.spec.ts b/test/integration/proxying/request-buffering.spec.ts
new file mode 100644
index 000000000..87929716a
--- /dev/null
+++ b/test/integration/proxying/request-buffering.spec.ts
@@ -0,0 +1,241 @@
+import { Buffer } from 'buffer';
+import * as http from 'http';
+import * as http2 from 'http2';
+
+import { getLocal, Mockttp, MockttpOptions, CompletedRequest, BodyData } from '../../..';
+import { expect, nodeOnly, getDeferred, Deferred, makeDestroyable, DestroyableServer } from '../../test-utils';
+import type { BufferInProgress } from '../../../src/util/buffer-utils';
+
+nodeOnly(() => {
+    // Package-root imports run the built server, so instrument its body reader.
+    const bufferUtils: typeof import('../../../src/util/buffer-utils') = require('../../../dist/util/buffer-utils');
+    describe("Passthrough request buffering", () => {
+        let proxy: Mockttp | undefined;
+        let target: DestroyableServer<http.Server>;
+        let targetUrl: string;
+        let received: Deferred<{ body: Buffer, trailers: http.IncomingHttpHeaders }>;
+        let releaseResponse: Deferred<void>;
+        let firstChunk: Deferred<void>;
+        let upstreamAborted: Deferred<void>;
+        let captures: BufferInProgress[];
+        let bufferDescriptor: PropertyDescriptor;
+
+        beforeEach(async () => {
+            received = getDeferred();
+            releaseResponse = getDeferred();
+            firstChunk = getDeferred();
+            upstreamAborted = getDeferred();
+            captures = [];
+            bufferDescriptor = Object.getOwnPropertyDescriptor(bufferUtils, 'streamToBuffer')!;
+            const original = bufferUtils.streamToBuffer;
+            Object.defineProperty(bufferUtils, 'streamToBuffer', {
+                ...bufferDescriptor,
+                value: (...args: Parameters<typeof original>) => {
+                    const result = original(...args);
+                    if ((args[0] as http.IncomingMessage).method === 'POST') captures.push(result);
+                    return result;
+                }
+            });
+            target = makeDestroyable(http.createServer((request, response) => {
+                const chunks: Buffer[] = [];
+                request.on('data', chunk => {
+                    chunks.push(chunk);
+                    firstChunk.resolve();
+                });
+                request.on('aborted', () => upstreamAborted.resolve());
+                request.on('end', () => {
+                    received.resolve({ body: Buffer.concat(chunks), trailers: request.trailers });
+                    releaseResponse.then(() => response.end('ok'));
+                });
+            }));
+            await new Promise<void>(resolve => target.listen(0, '127.0.0.1', resolve));
+            targetUrl = `http://127.0.0.1:${(target.address() as { port: number }).port}`;
+        });
+
+        afterEach(async () => {
+            releaseResponse.resolve();
+            await proxy?.stop();
+            proxy = undefined;
+            await target.destroy();
+            Object.defineProperty(bufferUtils, 'streamToBuffer', bufferDescriptor);
+        });
+
+        async function start(options: MockttpOptions = { recordTraffic: false }) {
+            proxy = getLocal(options);
+            await proxy.start();
+            return proxy;
+        }
+
+        function upload(body: Buffer | string, trailers?: Record<string, string>) {
+            const finished = getDeferred<void>();
+            const request = http.request(proxy!.urlFor('/upload'), {
+                method: 'POST',
+                path: `${targetUrl}/upload`,
+                headers: {
+                    Host: new URL(targetUrl).host,
+                    ...(trailers ? { Trailer: Object.keys(trailers).join(', ') } : {})
+                }
+            }, response => {
+                response.resume();
+                response.on('end', () => finished.resolve());
+                response.on('error', finished.reject);
+            });
+            request.on('error', finished.reject);
+            request.write(body);
+            if (trailers) request.addTrailers(trailers);
+            request.end();
+            // A failed assertion can close the proxy before this request finishes.
+            finished.catch(() => {});
+            return finished;
+        }
+
+        const retainedBytes = () => captures.reduce((total, buffer) => total +
+            buffer.currentChunks.reduce((size, chunk) => size + chunk.length, 0), 0);
+
+        it("forwards an unobserved upload without retaining its body", async () => {
+            const server = await start();
+            const endpoint = await server.forAnyRequest().thenForwardTo(targetUrl);
+            const body = Buffer.alloc(1024 * 1024, 'a');
+            const finished = upload(body, { 'X-Upload-End': 'complete' });
+            const actual = await received;
+
+            expect(actual.body).to.deep.equal(body);
+            expect(actual.trailers).to.deep.equal({ 'x-upload-end': 'complete' });
+            expect(retainedBytes()).to.equal(0);
+            releaseResponse.resolve();
+            await finished;
+            expect(await endpoint.getSeenRequests()).to.deep.equal([]);
+        });
+
+        it("preserves bodies for traffic recording", async () => {
+            const server = await start({ recordTraffic: true });
+            const endpoint = await server.forAnyRequest().thenForwardTo(targetUrl);
+            const finished = upload('recorded upload');
+            expect((await received).body.toString()).to.equal('recorded upload');
+            releaseResponse.resolve();
+            await finished;
+            const requests = await endpoint.getSeenRequests();
+            expect(await requests[0].body.getText()).to.equal('recorded upload');
+        });
+
+        it("streams HTTP/2 uploads without retaining a replay buffer", async () => {
+            const server = await start({
+                recordTraffic: false,
+                http2: true,
+                https: {
+                    keyPath: './test/fixtures/test-ca.key',
+                    certPath: './test/fixtures/test-ca.pem'
+                }
+            });
+            await server.forAnyRequest().thenForwardTo(targetUrl);
+            const client = http2.connect(server.url);
+            try {
+                const request = client.request({ ':method': 'POST', ':path': '/upload' });
+                const finished = getDeferred<void>();
+                request.on('error', finished.reject);
+                request.on('end', () => finished.resolve());
+                finished.catch(() => {});
+                request.resume();
+                const body = Buffer.alloc(256 * 1024, 'h');
+                request.end(body);
+                expect((await received).body).to.deep.equal(body);
+                expect(retainedBytes()).to.equal(0);
+                releaseResponse.resolve();
+                await finished;
+            } finally {
+                client.destroy();
+            }
+        });
+
+        it("cancels an unbuffered upstream upload when the client disconnects", async () => {
+            const server = await start();
+            await server.forAnyRequest().thenForwardTo(targetUrl);
+            const request = http.request(server.urlFor('/upload'), { method: 'POST' });
+            request.on('error', () => {});
+            request.write(Buffer.alloc(64 * 1024, 'a'));
+            await firstChunk;
+            request.destroy();
+            await upstreamAborted;
+        });
+
+        it("preserves bodies for complete request subscriptions", async () => {
+            const server = await start();
+            const observed = getDeferred<CompletedRequest>();
+            await server.on('request', request => observed.resolve(request));
+            await server.forAnyRequest().thenForwardTo(targetUrl);
+            const finished = upload('observed upload');
+            expect((await received).body.toString()).to.equal('observed upload');
+            expect(await (await observed).body.getText()).to.equal('observed upload');
+            releaseResponse.resolve();
+            await finished;
+        });
+
+        it("preserves streaming request data subscriptions", async () => {
+            const server = await start();
+            const events: BodyData[] = [];
+            const ended = getDeferred<void>();
+            await server.on('request-body-data', event => {
+                events.push(event);
+                if (event.isEnded) ended.resolve();
+            });
+            await server.forAnyRequest().thenForwardTo(targetUrl);
+            const body = Buffer.alloc(256 * 1024, 'b');
+            const finished = upload(body);
+            expect((await received).body).to.deep.equal(body);
+            await ended;
+            expect(Buffer.concat(events.map(event => Buffer.from(event.content)))).to.deep.equal(body);
+            releaseResponse.resolve();
+            await finished;
+        });
+
+        it("replays bodies already read by a matcher", async () => {
+            const server = await start();
+            await server.forAnyRequest().withBody('matched upload').thenForwardTo(targetUrl);
+            const finished = upload('matched upload');
+            expect((await received).body.toString()).to.equal('matched upload');
+            releaseResponse.resolve();
+            await finished;
+        });
+
+        for (const callback of ['beforeRequest', 'beforeResponse'] as const) {
+            it(`preserves bodies needed by ${callback}`, async () => {
+                const server = await start();
+                let observed: string | undefined;
+                await server.forAnyRequest().thenPassThrough(callback === 'beforeRequest' ? {
+                    beforeRequest: async request => { observed = await request.body.getText(); }
+                } : {
+                    beforeResponse: async (_response, request) => { observed = await request.body.getText(); }
+                });
+                const finished = upload('callback upload');
+                expect((await received).body.toString()).to.equal('callback upload');
+                releaseResponse.resolve();
+                await finished;
+                expect(observed).to.equal('callback upload');
+            });
+        }
+
+        it("preserves request body transforms", async () => {
+            const server = await start();
+            await server.forAnyRequest().thenForwardTo(targetUrl, {
+                transformRequest: { updateJsonBody: { extra: true } }
+            });
+            const finished = upload(JSON.stringify({ original: true }));
+            expect(JSON.parse((await received).body.toString())).to.deep.equal({ original: true, extra: true });
+            releaseResponse.resolve();
+            await finished;
+        });
+
+        it("forwards oversized observed uploads without losing their prefix", async () => {
+            const server = await start({ recordTraffic: false, maxBodySize: 4 });
+            const observed = getDeferred<CompletedRequest>();
+            await server.on('request', request => observed.resolve(request));
+            await server.forAnyRequest().thenForwardTo(targetUrl);
+            const body = Buffer.alloc(256 * 1024, 'c');
+            const finished = upload(body);
+            expect((await received).body).to.deep.equal(body);
+            expect((await observed).body.buffer.length).to.equal(0);
+            releaseResponse.resolve();
+            await finished;
+        });
+    });
+});