diff --git a/synapse/storage/_base.py b/synapse/storage/_base.py
index 78ba5f25ea..4b1ec687c9 100644
--- a/synapse/storage/_base.py
+++ b/synapse/storage/_base.py
@@ -48,24 +48,30 @@ def cached(max_entries=1000):
 
     The wrapped function has an additional member, a callable called
     "invalidate". This can be used to remove individual entries from the cache.
+
+    The wrapped function has another additional callable, called "prefill",
+    which can be used to insert values into the cache specifically, without
+    calling the calculation function.
     """
     def wrap(orig):
         cache = {}
 
+        def prefill(key, value):
+            while len(cache) > max_entries:
+                # TODO(paul): This feels too biased. However, a random index
+                #   would be a bit inefficient, walking the list of keys just
+                #   to ignore most of them?
+                del cache[cache.keys()[0]]
+
+            cache[key] = value
+
         @defer.inlineCallbacks
         def wrapped(self, key):
             if key in cache:
                 defer.returnValue(cache[key])
 
             ret = yield orig(self, key)
-
-            while len(cache) > max_entries:
-                # TODO(paul): This feels too biased. However, a random index
-                #   would be a bit inefficient, walking the list of keys just
-                #   to ignore most of them?
-                del cache[cache.keys()[0]]
-
-            cache[key] = ret;
+            prefill(key, ret)
             defer.returnValue(ret)
 
         def invalidate(key):
@@ -73,6 +79,7 @@ def cached(max_entries=1000):
                 del cache[key]
 
         wrapped.invalidate = invalidate
+        wrapped.prefill    = prefill
         return wrapped
 
     return wrap
diff --git a/tests/storage/test__base.py b/tests/storage/test__base.py
index 057f798640..fb306cb784 100644
--- a/tests/storage/test__base.py
+++ b/tests/storage/test__base.py
@@ -87,3 +87,17 @@ class CacheDecoratorTestCase(unittest.TestCase):
 
         self.assertTrue(callcount[0] >= 14,
             msg="Expected callcount >= 14, got %d" % (callcount[0]))
+
+    @defer.inlineCallbacks
+    def test_prefill(self):
+        callcount = [0]
+
+        @cached()
+        def func(self, key):
+            callcount[0] += 1
+            return key
+
+        func.prefill("foo", 123)
+
+        self.assertEquals((yield func(self, "foo")), 123)
+        self.assertEquals(callcount[0], 0)