diff --git a/synapse/storage/roommember.py b/synapse/storage/roommember.py
index d490a374e..e05465bc1 100644
--- a/synapse/storage/roommember.py
+++ b/synapse/storage/roommember.py
@@ -35,6 +35,11 @@ RoomsForUser = namedtuple(
 
 class RoomMemberStore(SQLBaseStore):
 
+    def __init__(self, *args, **kw):
+        super(RoomMemberStore, self).__init__(*args, **kw)
+
+        self._user_rooms_cache = {}
+
     def _store_room_member_txn(self, txn, event):
         """Store a room member in the database.
         """
@@ -98,6 +103,8 @@ class RoomMemberStore(SQLBaseStore):
 
                 txn.execute(sql, (event.room_id, domain))
 
+        self.invalidate_rooms_for_user(target_user_id)
+
     @defer.inlineCallbacks
     def get_room_member(self, user_id, room_id):
         """Retrieve the current state of a room member.
@@ -240,23 +247,43 @@ class RoomMemberStore(SQLBaseStore):
         results = self._parse_events_txn(txn, rows)
         return results
 
+    # TODO(paul): Create a nice @cached decorator to do this
+    #    @cached
+    #    def get_foo(...)
+    #        ...
+    #    invalidate_foo = get_foo.invalidator
+
+    @defer.inlineCallbacks
+    def get_rooms_for_user(self, user_id):
+        # TODO(paul): put some performance counters in here so we can easily
+        #   track what impact this cache is having
+        if user_id in self._user_rooms_cache:
+            defer.returnValue(self._user_rooms_cache[user_id])
+
+        rooms = yield self.get_rooms_for_user_where_membership_is(
+            user_id, membership_list=[Membership.JOIN],
+        )
+
+        self._user_rooms_cache[user_id] = rooms
+        defer.returnValue(rooms)
+
+    def invalidate_rooms_for_user(self, user_id):
+        if user_id in self._user_rooms_cache:
+            del self._user_rooms_cache[user_id]
+
     @defer.inlineCallbacks
     def user_rooms_intersect(self, user_id_list):
         """ Checks whether all the users whose IDs are given in a list share a
         room.
 
         This is a "hot path" function that's called a lot, e.g. by presence for
-        generating the event stream.
+        generating the event stream. As such, it is implemented locally by
+        wrapping logic around heavily-cached database queries.
         """
         if len(user_id_list) < 2:
             defer.returnValue(True)
 
-        deferreds = [
-            self.get_rooms_for_user_where_membership_is(
-                u, membership_list=[Membership.JOIN],
-            )
-            for u in user_id_list
-        ]
+        deferreds = [self.get_rooms_for_user(u) for u in user_id_list]
 
         results = yield defer.DeferredList(deferreds)