diff --git a/services/track-changes/app/js/HttpController.js b/services/track-changes/app/js/HttpController.js
index 54191fd909..c5db48210a 100644
--- a/services/track-changes/app/js/HttpController.js
+++ b/services/track-changes/app/js/HttpController.js
@@ -209,7 +209,7 @@ module.exports = HttpController = {
     logger.log({ project_id }, 'exporting project history')
     UpdatesManager.exportProject(project_id, function (
       err,
-      updates,
+      { updates, userIds },
       confirmWrite
     ) {
       const abortStreaming = req.aborted || res.finished || res.destroyed
@@ -236,6 +236,7 @@ module.exports = HttpController = {
         // The first write will emit the 200 status, headers and start of the
         //  response payload (open array)
         res.setHeader('Content-Type', 'application/json')
+        res.setHeader('Trailer', 'X-User-Ids')
         res.writeHead(200)
         res.write('[')
       }
@@ -255,7 +256,8 @@ module.exports = HttpController = {
 
       if (isLastWrite) {
         // The last write will have no updates and will finish the response
-        //  payload (close array).
+        //  payload (close array) and emit the userIds as trailer.
+        res.addTrailers({ 'X-User-Ids': JSON.stringify(userIds) })
         res.end(']')
       }
     })
diff --git a/services/track-changes/app/js/UpdatesManager.js b/services/track-changes/app/js/UpdatesManager.js
index 8e0e2dc569..7890208788 100644
--- a/services/track-changes/app/js/UpdatesManager.js
+++ b/services/track-changes/app/js/UpdatesManager.js
@@ -641,6 +641,8 @@ module.exports = UpdatesManager = {
       PackManager.makeProjectIterator(projectId, before, (err, iterator) => {
         if (err) return consumer(err)
 
+        const accumulatedUserIds = new Set()
+
         async.whilst(
           () => !iterator.done(),
 
@@ -654,13 +656,26 @@ module.exports = UpdatesManager = {
                 //  call.
                 return cb()
               }
+              updatesFromASinglePack.forEach((update) => {
+                accumulatedUserIds.add(
+                  // Super defensive access on update details.
+                  String(update && update.meta && update.meta.user_id)
+                )
+              })
               // Emit updates and wait for the consumer.
-              consumer(null, updatesFromASinglePack, cb)
+              consumer(null, { updates: updatesFromASinglePack }, cb)
             }),
 
           (err) => {
             if (err) return consumer(err)
-            consumer(null, [])
+
+            // Adding undefined can happen for broken updates.
+            accumulatedUserIds.delete('undefined')
+
+            consumer(null, {
+              updates: [],
+              userIds: Array.from(accumulatedUserIds).sort()
+            })
           }
         )
       })
diff --git a/services/track-changes/test/acceptance/js/ArchivingUpdatesTests.js b/services/track-changes/test/acceptance/js/ArchivingUpdatesTests.js
index d082634573..bb1024389c 100644
--- a/services/track-changes/test/acceptance/js/ArchivingUpdatesTests.js
+++ b/services/track-changes/test/acceptance/js/ArchivingUpdatesTests.js
@@ -50,6 +50,7 @@ describe('Archiving updates', function () {
     this.now = Date.now()
     this.to = this.now
     this.user_id = ObjectId().toString()
+    this.user_id_2 = ObjectId().toString()
     this.doc_id = ObjectId().toString()
     this.project_id = ObjectId().toString()
 
@@ -92,7 +93,7 @@ describe('Archiving updates', function () {
         op: [{ i: 'b', p: 0 }],
         meta: {
           ts: this.now + (i - 2048) * this.hours + 10 * this.minutes,
-          user_id: this.user_id
+          user_id: this.user_id_2
         },
         v: 2 * i + 2
       })
@@ -144,13 +145,17 @@ describe('Archiving updates', function () {
   function testExportFeature() {
     describe('exporting the project', function () {
       before('fetch export', function (done) {
-        TrackChangesClient.exportProject(this.project_id, (error, updates) => {
-          if (error) {
-            return done(error)
+        TrackChangesClient.exportProject(
+          this.project_id,
+          (error, updates, userIds) => {
+            if (error) {
+              return done(error)
+            }
+            this.exportedUpdates = updates
+            this.exportedUserIds = userIds
+            done()
           }
-          this.exportedUpdates = updates
-          done()
-        })
+        )
       })
 
       it('should include all the imported updates, with ids, sorted by timestamp', function () {
@@ -175,6 +180,10 @@ describe('Archiving updates', function () {
             return exportedUpdate
           })
         expect(this.exportedUpdates).to.deep.equal(expectedExportedUpdates)
+        expect(this.exportedUserIds).to.deep.equal([
+          this.user_id,
+          this.user_id_2
+        ])
       })
     })
   }
diff --git a/services/track-changes/test/acceptance/js/ExportProjectTests.js b/services/track-changes/test/acceptance/js/ExportProjectTests.js
index d2958d7fc0..b6ca106a60 100644
--- a/services/track-changes/test/acceptance/js/ExportProjectTests.js
+++ b/services/track-changes/test/acceptance/js/ExportProjectTests.js
@@ -11,17 +11,22 @@ describe('ExportProject', function () {
 
   describe('when there are no updates', function () {
     before('fetch export', function (done) {
-      TrackChangesClient.exportProject(ObjectId(), (error, updates) => {
-        if (error) {
-          return done(error)
+      TrackChangesClient.exportProject(
+        ObjectId(),
+        (error, updates, userIds) => {
+          if (error) {
+            return done(error)
+          }
+          this.exportedUpdates = updates
+          this.exportedUserIds = userIds
+          done()
         }
-        this.exportedUpdates = updates
-        done()
-      })
+      )
     })
 
     it('should export an empty array', function () {
       expect(this.exportedUpdates).to.deep.equal([])
+      expect(this.exportedUserIds).to.deep.equal([])
     })
   })
 
diff --git a/services/track-changes/test/acceptance/js/helpers/TrackChangesClient.js b/services/track-changes/test/acceptance/js/helpers/TrackChangesClient.js
index aa25839341..f20884448d 100644
--- a/services/track-changes/test/acceptance/js/helpers/TrackChangesClient.js
+++ b/services/track-changes/test/acceptance/js/helpers/TrackChangesClient.js
@@ -171,7 +171,7 @@ module.exports = TrackChangesClient = {
       (error, response, updates) => {
         if (error) return callback(error)
         response.statusCode.should.equal(200)
-        callback(null, updates)
+        callback(null, updates, JSON.parse(response.trailers['x-user-ids']))
       }
     )
   },