diff --git a/synapse/handlers/presence.py b/synapse/handlers/presence.py
index 28e922f79..698946a48 100644
--- a/synapse/handlers/presence.py
+++ b/synapse/handlers/presence.py
@@ -21,6 +21,7 @@ from synapse.api.constants import PresenceState
 from synapse.util.logutils import log_function
 from synapse.util.logcontext import PreserveLoggingContext
 from synapse.types import UserID
+import synapse.metrics
 
 from ._base import BaseHandler
 
@@ -29,6 +30,8 @@ import logging
 
 logger = logging.getLogger(__name__)
 
+metrics = synapse.metrics.get_metrics_for(__name__)
+
 
 # TODO(paul): Maybe there's one of these I can steal from somewhere
 def partition(l, func):
@@ -133,6 +136,10 @@ class PresenceHandler(BaseHandler):
         self._user_cachemap = {}
         self._user_cachemap_latest_serial = 0
 
+        metrics.register_callback("user_cachemap:size",
+            lambda: len(self._user_cachemap)
+        )
+
     def _get_or_make_usercache(self, user):
         """If the cache entry doesn't exist, initialise a new one."""
         if user not in self._user_cachemap: