diff --git a/src/client.js b/src/client.js
index 162f596d8..7bd607fd1 100644
--- a/src/client.js
+++ b/src/client.js
@@ -38,6 +38,7 @@ const Filter = require("./filter");
 const SyncApi = require("./sync");
 const MatrixBaseApis = require("./base-apis");
 const MatrixError = httpApi.MatrixError;
+const IndexedDBStore = require("./store/indexeddb").IndexedDBStore;
 
 const SCROLLBACK_DELAY_MS = 3000;
 let CRYPTO_ENABLED = false;
@@ -163,6 +164,11 @@ function MatrixClient(opts) {
 
         this.olmVersion = Crypto.getOlmVersion();
     }
+
+    // Set up a sync accumulator if we can persist room data
+    if (this.store instanceof IndexedDBStore) {
+        this._syncAccumulator = this.store.getSyncAccumulator();
+    }
 }
 utils.inherits(MatrixClient, EventEmitter);
 utils.extend(MatrixClient.prototype, MatrixBaseApis.prototype);
@@ -2695,6 +2701,7 @@ MatrixClient.prototype.startClient = function(opts) {
     opts = Object.assign({}, opts);
 
     opts.crypto = this._crypto;
+    opts.syncAccumulator = this._syncAccumulator;
     this._clientOpts = opts;
 
     this._syncApi = new SyncApi(this, opts);
diff --git a/src/matrix.js b/src/matrix.js
index b54501023..a3ea7ae45 100644
--- a/src/matrix.js
+++ b/src/matrix.js
@@ -25,6 +25,8 @@ module.exports.MatrixInMemoryStore = require("./store/memory").MatrixInMemorySto
 module.exports.IndexedDBStore = require("./store/indexeddb").IndexedDBStore;
 /** The {@link module:store/indexeddb.IndexedDBStoreBackend|IndexedDBStoreBackend} class. */
 module.exports.IndexedDBStoreBackend = require("./store/indexeddb").IndexedDBStoreBackend;
+/** The {@link module:sync-accumulator.SyncAccumulator|SyncAccumulator} class. */
+module.exports.SyncAccumulator = require("./sync-accumulator");
 /** The {@link module:http-api.MatrixHttpApi|MatrixHttpApi} class. */
 module.exports.MatrixHttpApi = require("./http-api").MatrixHttpApi;
 /** The {@link module:http-api.MatrixError|MatrixError} class. */
diff --git a/src/store/indexeddb.js b/src/store/indexeddb.js
index 1182aa489..8fa102eac 100644
--- a/src/store/indexeddb.js
+++ b/src/store/indexeddb.js
@@ -18,7 +18,6 @@ limitations under the License.
 import q from "q";
 import {MatrixInMemoryStore} from "./memory";
 import User from "../models/user";
-import Room from "../models/room";
 import {MatrixEvent} from "../models/event";
 import utils from "../utils";
 
@@ -77,25 +76,18 @@ IndexedDBStoreBackend.prototype = {
     },
 
     /**
-     * Persist a list of Room objects. Rooms with the same 'roomId' will be replaced.
-     * @param {Room[]} rooms An array of rooms
-     * @return {Promise} Resolves if the rooms were persisted.
+     * Persist rooms /sync data along with the next batch token.
+     * @param {string} nextBatch The next_batch /sync value.
+     * @param {Object} roomsData The 'rooms' /sync data from a SyncAccumulator
+     * @return {Promise} Resolves if the data was persisted.
      */
-    persistRooms: function(rooms) {
-        return this._upsert("rooms", rooms);
-    },
-
-    /**
-     * Persist a sync token. This will replace any existing sync token.
-     * @param {string} syncToken The token to persist.
-     * @return {Promise} Resolves if the token was persisted.
-     */
-    persistSyncToken: function(syncToken) {
+    persistSyncData: function(nextBatch, roomsData) {
         const obj = {
             clobber: "-", // constant key so will always clobber
-            syncToken: syncToken,
+            nextBatch: nextBatch,
+            roomsData: roomsData,
         };
-        return this._upsert("config", [obj]);
+        return this._upsert("sync", [obj]);
     },
 
     /**
@@ -134,14 +126,6 @@ IndexedDBStoreBackend.prototype = {
         return this._deserializeAll("users", User);
     },
 
-    /**
-     * Load all the rooms from the database. This is not cached.
-     * @return {Promise<Room[]>} A list of rooms.
-     */
-    loadRooms: function() {
-        return this._deserializeAll("rooms", Room);
-    },
-
     /**
      * Load all the account data events from the database. This is not cached.
      * @return {Promise<MatrixEvent[]>} A list of events.
@@ -157,20 +141,20 @@ IndexedDBStoreBackend.prototype = {
     },
 
     /**
-     * Load the sync token from the database.
-     * @return {Promise<?string>} The sync token
+     * Load the sync data from the database.
+     * @return {Promise<Object>} An object with "roomsData" and "nextBatch" keys.
      */
-    loadSyncToken: function() {
+    loadSyncData: function() {
         return q.try(() => {
-            const txn = this.db.transaction(["config"], "readonly");
-            const store = txn.objectStore("config");
+            const txn = this.db.transaction(["sync"], "readonly");
+            const store = txn.objectStore("sync");
             const results = selectQuery(store, undefined, (cursor) => {
                 return cursor.value;
             });
             if (results.length > 1) {
-                console.warn("loadSyncToken: More than 1 config row found.");
+                console.warn("loadSyncData: More than 1 sync row found.");
             }
-            return (results.length > 0 ? results[0].syncToken : null);
+            return (results.length > 0 ? results[0] : {});
         });
     },
 
@@ -211,8 +195,11 @@ IndexedDBStoreBackend.prototype = {
  * sync from the server is not required. This does not reduce memory usage as all
  * the data is eagerly fetched when <code>startup()</code> is called.
  * <pre>
+ * let syncAccumulator = new SyncAccumulator();
  * let opts = { localStorage: window.localStorage };
- * let store = new IndexedDBStore(new IndexedDBStoreBackend(window.indexedDB), opts);
+ * let store = new IndexedDBStore(
+ *     new IndexedDBStoreBackend(window.indexedDB), syncAccumulator, opts
+ * );
  * await store.startup(); // load from indexed db
  * let client = sdk.createClient({
  *     store: store,
@@ -228,12 +215,14 @@ IndexedDBStoreBackend.prototype = {
  * @constructor
  * @extends MatrixInMemoryStore
  * @param {IndexedDBStoreBackend} backend The indexed db backend instance.
+ * @param {SyncAccumulator} syncAccumulator The sync accumulator which will be
+ * loaded from IndexedDB and periodically saved to IndexedDB.
  * @param {Object=} opts Options for MatrixInMemoryStore.
  * @prop {IndexedDBStoreBackend} backend The backend instance. Call through to
  * this API if you need to perform specific indexeddb actions like deleting the
  * database.
  */
-const IndexedDBStore = function IndexedDBStore(backend, opts) {
+const IndexedDBStore = function IndexedDBStore(backend, syncAccumulator, opts) {
     MatrixInMemoryStore.call(this, opts);
     this.backend = backend;
     this.startedUp = false;
@@ -243,6 +232,11 @@ const IndexedDBStore = function IndexedDBStore(backend, opts) {
     this._userModifiedMap = {
         // user_id : timestamp
     };
+    this._syncAccumulator = syncAccumulator;
+
+    if (!this.backend || !this._syncAccumulator) {
+        throw new Error("Missing backend or syncAccumulator");
+    }
 };
 utils.inherits(IndexedDBStore, MatrixInMemoryStore);
 
@@ -258,10 +252,10 @@ IndexedDBStore.prototype.startup = function() {
             this.backend.loadUsers(),
             this.backend.loadAccountData(),
             this.backend.loadRooms(),
-            this.backend.loadSyncToken(),
+            this.backend.loadSyncData(),
         ]);
     }).then((values) => {
-        const [users, accountData, rooms, syncToken] = values;
+        const [users, accountData, rooms, syncData] = values;
         console.log(
             "Loaded data from database. Reticulating splines...",
             accountData, users,
@@ -275,10 +269,20 @@ IndexedDBStore.prototype.startup = function() {
             this.storeRoom(r);
         });
         this._syncTs = Date.now(); // pretend we've written so we don't rewrite
-        this.setSyncToken(syncToken);
+        this.setSyncToken(syncData.syncToken);
+        this._setSyncData(syncData.syncToken, syncData.roomsData);
     });
 };
 
+/**
+ * Return the accumulator which will have the initial /sync data when startup()
+ * is called.
+ * @return {SyncAccumulator}
+ */
+IndexedDBStore.prototype.getSyncAccumulator = function() {
+    return this._syncAccumulator;
+};
+
 /**
  * Set a new sync token and possibly write to the database.
  * Overrides MatrixInMemoryStore.
@@ -295,6 +299,13 @@ IndexedDBStore.prototype.setSyncToken = function(token) {
     return null;
 };
 
+IndexedDBStore.prototype._setSyncData = function(nextBatch, roomsData) {
+    this._syncAccumulator.accumulateRooms({
+        next_batch: nextBatch,
+        rooms: roomsData,
+    });
+};
+
 IndexedDBStore.prototype._syncToDatabase = function() {
     console.log("_syncToDatabase");
     this._syncTs = Date.now(); // set now to guard against multi-writes
@@ -315,16 +326,16 @@ IndexedDBStore.prototype._syncToDatabase = function() {
         return this.accountData[etype];
     });
 
+    const syncData = this._syncAccumulator.getJSON();
+
     return q.all([
         this.backend.persistUsers(changedUsers),
         this.backend.persistAccountData(changedAccountData),
+        this.backend.persistSyncData(syncData.nextBatch, syncData.roomsData),
     ]);
 };
 
 function createDatabase(db) {
-    // Make room store, clobber based on room ID. (roomId property of Room objects)
-    db.createObjectStore("rooms", { keyPath: ["roomId"] });
-
     // Make user store, clobber based on user ID. (userId property of User objects)
     db.createObjectStore("users", { keyPath: ["userId"] });
 
@@ -332,8 +343,8 @@ function createDatabase(db) {
     // (event.type property of MatrixEvent objects)
     db.createObjectStore("accountData", { keyPath: ["type"] });
 
-    // Make configuration store (sync tokens, etc), always clobber (const key).
-    db.createObjectStore("config", { keyPath: ["clobber"] });
+    // Make /sync store (sync tokens, room data, etc), always clobber (const key).
+    db.createObjectStore("sync", { keyPath: ["clobber"] });
 }
 
 /**
diff --git a/src/sync-accumulator.js b/src/sync-accumulator.js
index 9a608fd1b..65df31e07 100644
--- a/src/sync-accumulator.js
+++ b/src/sync-accumulator.js
@@ -15,10 +15,13 @@ limitations under the License.
 */
 "use strict";
 
+/**
+ * This is an internal module. See {@link SyncAccumulator} for the public class.
+ * @module sync-accumulator
+ */
+
 
 /**
- * Internal class.
- *
  * The purpose of this class is to accumulate /sync responses such that a
  * complete "initial" JSON response can be returned which accurately represents
  * the sum total of the /sync responses accumulated to date. It only handles
diff --git a/src/sync.js b/src/sync.js
index 474edc192..ef1150af9 100644
--- a/src/sync.js
+++ b/src/sync.js
@@ -60,7 +60,8 @@ function debuglog() {
  * @param {Object} opts Config options
  * @param {module:crypto=} opts.crypto Crypto manager
  * @param {SyncAccumulator=} opts.syncAccumulator An accumulator which will be
- * kept up-to-date.
+ * kept up-to-date. If one is supplied, the response to getJSON() will be used
+ * initially.
  */
 function SyncApi(client, opts) {
     this.client = client;
@@ -513,9 +514,26 @@ SyncApi.prototype._sync = function(syncOptions) {
     // normal timeout= plus buffer time
     const clientSideTimeoutMs = this.opts.pollTimeout + BUFFER_PERIOD_MS;
 
-    this._currentSyncRequest = client._http.authedRequest(
-        undefined, "GET", "/sync", qps, undefined, clientSideTimeoutMs,
-    );
+    let isCachedResponse = false;
+    if (self.opts.syncAccumulator && !syncOptions.hasSyncedBefore) {
+        const data = self.opts.syncAccumulator.getJSON();
+        // Don't do an HTTP hit to /sync. Instead, load up the persisted /sync data,
+        // if there is data there.
+        if (data.nextBatch) {
+            console.log("sync(): not doing HTTP hit, instead returning stored /sync");
+            this._currentSyncRequest = q.resolve({
+                next_batch: data.nextBatch,
+                rooms: data.roomsData,
+            });
+            isCachedResponse = true;
+        }
+    }
+
+    if (!isCachedResponse) {
+        this._currentSyncRequest = client._http.authedRequest(
+            undefined, "GET", "/sync", qps, undefined, clientSideTimeoutMs,
+        );
+    }
 
     this._currentSyncRequest.done(function(data) {
         // set the sync token NOW *before* processing the events. We do this so
@@ -531,7 +549,10 @@ SyncApi.prototype._sync = function(syncOptions) {
             console.error("Caught /sync error", e.stack || e);
         }
 
-        if(self.opts.syncAccumulator) {
+        // If there's an accumulator then the first HTTP response is actually the
+        // accumulated data. We don't want to accumulate the same thing twice, so
+        // only accumulate if this isn't a cached response.
+        if (!isCachedResponse) {
             self.opts.syncAccumulator.accumulateRooms(data);
         }