diff --git a/doorstop/gui/application.py b/doorstop/gui/application.py
index 83c20437e..31f1aba78 100644
--- a/doorstop/gui/application.py
+++ b/doorstop/gui/application.py
@@ -5,6 +5,7 @@
 """Graphical interface for Doorstop."""
 
 import functools
+import hashlib
 import logging
 import sys
 from itertools import chain
@@ -17,10 +18,11 @@
 
 try:
     import tkinter as tk
-    from tkinter import filedialog, ttk
+    from tkinter import filedialog, messagebox, ttk
 except ImportError as _exc:
     sys.stderr.write("WARNING: {}\n".format(_exc))
     tk = Mock()
+    messagebox = Mock()
     ttk = Mock()
 
 
@@ -55,6 +57,7 @@ def __init__(self, root, cwd, project):
         self.tree = None
         self.document = None
         self.item = None
+        self.item_file_fingerprint = None
 
         # Create string variables
         self.stringvar_project = tk.StringVar(value=project or "")
@@ -521,11 +524,15 @@ def display_item(self, *_):
             uid = self.stringvar_item.get()
             if uid == "":
                 self.item = None
+                self.item_file_fingerprint = None
             else:
                 try:
                     self.item = self.tree.find_item(uid)
                 except DoorstopError:
                     pass
+                else:
+                    self.item.load(reload=True)
+                    self.item_file_fingerprint = self._item_file_fingerprint()
             log.info("displaying item {}...".format(self.item))
 
             if uid != "":
@@ -643,6 +650,17 @@ def update_item(self, *_):
             logging.warning("no item selected")
             return
 
+        if self._item_changed_on_disk():
+            messagebox.showwarning(
+                "Item changed on disk",
+                "This item was modified outside Doorstop GUI. "
+                "The external version will be reloaded and this edit will not be saved.",
+            )
+            self.item.load(reload=True)
+            self.item_file_fingerprint = self._item_file_fingerprint()
+            self.display_item()
+            return
+
         # Update the current item
         log.info("updating {}...".format(self.item))
         self.item.auto = False
@@ -658,10 +676,25 @@ def update_item(self, *_):
         if name:
             self.item.set(name, self.stringvar_extendedvalue.get())
         self.item.save()
+        self.item_file_fingerprint = self._item_file_fingerprint()
 
         # Re-select this item
         self.display_document()
 
+    def _item_file_fingerprint(self):
+        """Return a content fingerprint for the current item's file."""
+        if self.item is None:
+            return None
+        try:
+            with open(self.item.path, "rb") as stream:
+                return hashlib.sha256(stream.read()).digest()
+        except OSError:
+            return None
+
+    def _item_changed_on_disk(self):
+        """Return whether the selected item's file changed since it was displayed."""
+        return self.item_file_fingerprint != self._item_file_fingerprint()
+
     @_log
     def left(self):
         """Dedent the current item's level."""
diff --git a/doorstop/gui/tests/test_application.py b/doorstop/gui/tests/test_application.py
new file mode 100644
index 000000000..5fe61ce60
--- /dev/null
+++ b/doorstop/gui/tests/test_application.py
@@ -0,0 +1,70 @@
+# SPDX-License-Identifier: LGPL-3.0-only
+# pylint: disable=protected-access
+
+"""Unit tests for the Doorstop GUI application."""
+
+from pathlib import Path
+from types import SimpleNamespace
+from unittest.mock import Mock, patch
+
+from doorstop.gui.application import Application
+
+
+def application_for(path: Path) -> Application:
+    """Create the portion of an application needed for file-state tests."""
+    application = object.__new__(Application)
+    application.item = SimpleNamespace(path=str(path))
+    application.item_file_fingerprint = None
+    return application
+
+
+def test_item_file_fingerprint_changes_with_content(tmp_path):
+    """Verify item fingerprints reflect the complete on-disk content."""
+    path = tmp_path / "REQ001.yml"
+    path.write_text("text: original\n", encoding="utf-8")
+    application = application_for(path)
+
+    original = application._item_file_fingerprint()
+    path.write_text("text: external update\n", encoding="utf-8")
+
+    assert original != application._item_file_fingerprint()
+
+
+def test_item_changed_on_disk(tmp_path):
+    """Verify an external item edit is detected after display."""
+    path = tmp_path / "REQ001.yml"
+    path.write_text("text: original\n", encoding="utf-8")
+    application = application_for(path)
+    application.item_file_fingerprint = application._item_file_fingerprint()
+
+    assert not application._item_changed_on_disk()
+
+    path.write_text("text: external update\n", encoding="utf-8")
+
+    assert application._item_changed_on_disk()
+
+
+def test_missing_item_file_has_no_fingerprint(tmp_path):
+    """Verify a missing item file does not raise an exception."""
+    application = application_for(tmp_path / "REQ001.yml")
+
+    assert application._item_file_fingerprint() is None
+
+
+@patch("doorstop.gui.application.messagebox.showwarning")
+def test_external_change_blocks_save(showwarning, tmp_path):
+    """Verify a stale GUI item cannot overwrite an external edit."""
+    path = tmp_path / "REQ001.yml"
+    path.write_text("text: original\n", encoding="utf-8")
+    application = application_for(path)
+    application.ignore = False
+    application.item = Mock(path=str(path))
+    application.item_file_fingerprint = b"stale"
+    application.display_item = Mock()
+
+    application.update_item()
+
+    showwarning.assert_called_once()
+    application.item.load.assert_called_once_with(reload=True)
+    application.item.save.assert_not_called()
+    application.display_item.assert_called_once_with()