diff --git a/synapse/storage/roommember.py b/synapse/storage/roommember.py
index 4db07f6fb..55dd3f6cf 100644
--- a/synapse/storage/roommember.py
+++ b/synapse/storage/roommember.py
@@ -78,7 +78,7 @@ class RoomMemberStore(SQLBaseStore):
             lambda events: events[0] if events else None
         )
 
-    @cached()
+    @cached(max_entries=5000)
     def get_users_in_room(self, room_id):
         def f(txn):
 
@@ -154,7 +154,7 @@ class RoomMemberStore(SQLBaseStore):
             RoomsForUser(**r) for r in self.cursor_to_dict(txn)
         ]
 
-    @cached()
+    @cached(max_entries=5000)
     def get_joined_hosts_for_room(self, room_id):
         return self.runInteraction(
             "get_joined_hosts_for_room",