diff --git a/src/sync-accumulator.js b/src/sync-accumulator.js
index 942c319eb..786841318 100644
--- a/src/sync-accumulator.js
+++ b/src/sync-accumulator.js
@@ -57,7 +57,8 @@ class SyncAccumulator {
             //       { event: $event, token: null|token },
             //       ...
             //    ],
-            //    _accountData: { $event_type: json }
+            //    _accountData: { $event_type: json },
+            //    _unreadNotifications: { ... unread_notifications JSON ... }
             //}
         };
         // the /sync token which corresponds to the last time rooms were
@@ -218,6 +219,7 @@ class SyncAccumulator {
                 _currentState: Object.create(null),
                 _timeline: [],
                 _accountData: Object.create(null),
+                _unreadNotifications: {},
             };
         }
         const currentData = this.joinRooms[roomId];
@@ -229,6 +231,11 @@ class SyncAccumulator {
             });
         }
 
+        // these probably clobber, spec is unclear.
+        if (data.unread_notifications) {
+            currentData._unreadNotifications = data.unread_notifications;
+        }
+
         // Work out the current state. The deltas need to be applied in the order:
         // - existing state which didn't come down /sync.
         // - State events under the 'state' key.
@@ -306,6 +313,7 @@ class SyncAccumulator {
                     events: [],
                     prev_batch: null,
                 },
+                unread_notifications: roomData._unreadNotifications,
             };
             // Add account data
             Object.keys(roomData._accountData).forEach((evType) => {