diff --git a/packages/metro-bundler/src/DeltaBundler/DeltaPatcher.js b/packages/metro-bundler/src/DeltaBundler/DeltaPatcher.js
index aafeec84..9416a5b6 100644
--- a/packages/metro-bundler/src/DeltaBundler/DeltaPatcher.js
+++ b/packages/metro-bundler/src/DeltaBundler/DeltaPatcher.js
@@ -28,6 +28,7 @@ class DeltaPatcher {
     modules: new Map(),
   };
   _initialized = false;
+  _lastNumModifiedFiles = 0;
 
   /**
    * Applies a Delta Bundle to the current bundle.
@@ -51,6 +52,9 @@ class DeltaPatcher {
       };
     }
 
+    this._lastNumModifiedFiles =
+      deltaBundle.pre.size + deltaBundle.post.size + deltaBundle.delta.size;
+
     this._patchMap(this._lastBundle.pre, deltaBundle.pre);
     this._patchMap(this._lastBundle.post, deltaBundle.post);
     this._patchMap(this._lastBundle.modules, deltaBundle.delta);
@@ -58,17 +62,27 @@ class DeltaPatcher {
     return this;
   }
 
+  /**
+   * Returns the number of modified files in the last received Delta. This is
+   * currently used to populate the `X-Metro-Files-Changed-Count` HTTP header
+   * when metro serves the whole JS bundle, and can potentially be removed once
+   * we only send the actual deltas to clients.
+   */
+  getLastNumModifiedFiles(): number {
+    return this._lastNumModifiedFiles;
+  }
+
   /**
    * Converts the current delta bundle to a standard string bundle, ready to
    * be interpreted by any JS VM.
    */
-  stringifyCode() {
+  stringifyCode(): string {
     const code = this._getAllModules().map(m => m.code);
 
     return code.join('\n;');
   }
 
-  stringifyMap({excludeSource}: {excludeSource?: boolean}) {
+  stringifyMap({excludeSource}: {excludeSource?: boolean}): string {
     const mappings = fromRawMappings(this._getAllModules());
 
     return mappings.toString(undefined, {excludeSource});
diff --git a/packages/metro-bundler/src/DeltaBundler/__tests__/DeltaPatcher-test.js b/packages/metro-bundler/src/DeltaBundler/__tests__/DeltaPatcher-test.js
index 1d9a3963..8dfbdd54 100644
--- a/packages/metro-bundler/src/DeltaBundler/__tests__/DeltaPatcher-test.js
+++ b/packages/metro-bundler/src/DeltaBundler/__tests__/DeltaPatcher-test.js
@@ -92,4 +92,29 @@ describe('DeltaPatcher', () => {
         .stringifyCode(),
     ).toMatchSnapshot();
   });
+
+  it('should return the number of modified files in the last Delta', () => {
+    deltaPatcher
+      .applyDelta({
+        reset: 1,
+        pre: new Map([[1, {code: 'pre'}]]),
+        post: new Map([[2, {code: 'post'}]]),
+        delta: new Map([[3, {code: 'middle'}]]),
+      })
+      .stringifyCode();
+
+    expect(deltaPatcher.getLastNumModifiedFiles()).toEqual(3);
+
+    deltaPatcher
+      .applyDelta({
+        reset: 1,
+        pre: new Map([[1, null]]),
+        post: new Map(),
+        delta: new Map([[3, {code: 'different'}]]),
+      })
+      .stringifyCode();
+
+    // A deleted module counts as a modified file.
+    expect(deltaPatcher.getLastNumModifiedFiles()).toEqual(2);
+  });
 });
diff --git a/packages/metro-bundler/src/DeltaBundler/index.js b/packages/metro-bundler/src/DeltaBundler/index.js
index 80e09df1..ea9c2b19 100644
--- a/packages/metro-bundler/src/DeltaBundler/index.js
+++ b/packages/metro-bundler/src/DeltaBundler/index.js
@@ -105,14 +105,20 @@ class DeltaBundler {
     };
   }
 
-  async buildFullBundle(options: FullBuildOptions): Promise<string> {
-    let output = (await this._getDeltaPatcher(options)).stringifyCode();
+  async buildFullBundle(
+    options: FullBuildOptions,
+  ): Promise<{bundle: string, numModifiedFiles: number}> {
+    const deltaPatcher = await this._getDeltaPatcher(options);
+    let bundle = deltaPatcher.stringifyCode();
 
     if (options.sourceMapUrl) {
-      output += '//# sourceMappingURL=' + options.sourceMapUrl;
+      bundle += '//# sourceMappingURL=' + options.sourceMapUrl;
     }
 
-    return output;
+    return {
+      bundle,
+      numModifiedFiles: deltaPatcher.getLastNumModifiedFiles(),
+    };
   }
 
   async buildFullSourceMap(options: FullBuildOptions): Promise<string> {
diff --git a/packages/metro-bundler/src/Server/index.js b/packages/metro-bundler/src/Server/index.js
index bbd4899c..d746fcf6 100644
--- a/packages/metro-bundler/src/Server/index.js
+++ b/packages/metro-bundler/src/Server/index.js
@@ -924,12 +924,13 @@ class Server {
     );
 
     let bundle;
+    let numModifiedFiles;
 
     try {
-      bundle = await this._deltaBundler.buildFullBundle({
+      ({bundle, numModifiedFiles} = await this._deltaBundler.buildFullBundle({
         ...options,
         deltaBundleId: this.optionsHash(options),
-      });
+      }));
     } catch (error) {
       this._handleError(res, this.optionsHash(options), error);
 
@@ -947,22 +948,24 @@ class Server {
       debug('Responding with 304');
       res.writeHead(304);
       res.end();
-
-      return;
+    } else {
+      res.setHeader(FILES_CHANGED_COUNT_HEADER, String(numModifiedFiles));
+      res.setHeader('Content-Type', 'application/javascript');
+      res.setHeader('ETag', etag);
+      res.setHeader('Content-Length', String(Buffer.byteLength(bundle)));
+      res.end(bundle);
     }
 
-    res.setHeader('Content-Type', 'application/javascript');
-    res.setHeader('ETag', etag);
-    res.setHeader('Content-Length', String(Buffer.byteLength(bundle)));
-    res.end(bundle);
-
     this._reporter.update({
       buildID,
       type: 'bundle_build_done',
     });
 
     debug('Finished response');
-    log(createActionEndEntry(requestingBundleLogEntry));
+    log({
+      ...createActionEndEntry(requestingBundleLogEntry),
+      outdated_modules: numModifiedFiles,
+    });
   }
 
   async _processSourceMapUsingDeltaBundler(