diff --git a/changelog.d/5750.misc b/changelog.d/5750.misc
new file mode 100644
index 0000000000..6beaa460a5
--- /dev/null
+++ b/changelog.d/5750.misc
@@ -0,0 +1 @@
+Add a prometheus metric for pending cache lookups.
\ No newline at end of file
diff --git a/synapse/util/caches/__init__.py b/synapse/util/caches/__init__.py
index 8271229015..b50e3503f0 100644
--- a/synapse/util/caches/__init__.py
+++ b/synapse/util/caches/__init__.py
@@ -1,5 +1,6 @@
 # -*- coding: utf-8 -*-
 # Copyright 2015, 2016 OpenMarket Ltd
+# Copyright 2019 The Matrix.org Foundation C.I.C.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -51,7 +52,19 @@ response_cache_evicted = Gauge(
 response_cache_total = Gauge("synapse_util_caches_response_cache:total", "", ["name"])
 
 
-def register_cache(cache_type, cache_name, cache):
+def register_cache(cache_type, cache_name, cache, collect_callback=None):
+    """Register a cache object for metric collection.
+
+    Args:
+        cache_type (str):
+        cache_name (str): name of the cache
+        cache (object): cache itself
+        collect_callback (callable|None): if not None, a function which is called during
+            metric collection to update additional metrics.
+
+    Returns:
+        CacheMetric: an object which provides inc_{hits,misses,evictions} methods
+    """
 
     # Check if the metric is already registered. Unregister it, if so.
     # This usually happens during tests, as at runtime these caches are
@@ -90,6 +103,8 @@ def register_cache(cache_type, cache_name, cache):
                     cache_hits.labels(cache_name).set(self.hits)
                     cache_evicted.labels(cache_name).set(self.evicted_size)
                     cache_total.labels(cache_name).set(self.hits + self.misses)
+                if collect_callback:
+                    collect_callback()
             except Exception as e:
                 logger.warn("Error calculating metrics for %s: %s", cache_name, e)
                 raise
diff --git a/synapse/util/caches/descriptors.py b/synapse/util/caches/descriptors.py
index a1acacbde9..7e69cf55fb 100644
--- a/synapse/util/caches/descriptors.py
+++ b/synapse/util/caches/descriptors.py
@@ -22,6 +22,8 @@ from collections import namedtuple
 import six
 from six import itervalues, string_types
 
+from prometheus_client import Gauge
+
 from twisted.internet import defer
 
 from synapse.logging.context import make_deferred_yieldable, preserve_fn
@@ -37,6 +39,12 @@ from . import register_cache
 logger = logging.getLogger(__name__)
 
 
+cache_pending_metric = Gauge(
+    "synapse_util_caches_cache_pending",
+    "Number of lookups currently pending for this cache",
+    ["name"],
+)
+
 _CacheSentinel = object()
 
 
@@ -82,11 +90,19 @@ class Cache(object):
         self.name = name
         self.keylen = keylen
         self.thread = None
-        self.metrics = register_cache("cache", name, self.cache)
+        self.metrics = register_cache(
+            "cache",
+            name,
+            self.cache,
+            collect_callback=self._metrics_collection_callback,
+        )
 
     def _on_evicted(self, evicted_count):
         self.metrics.inc_evictions(evicted_count)
 
+    def _metrics_collection_callback(self):
+        cache_pending_metric.labels(self.name).set(len(self._pending_deferred_cache))
+
     def check_thread(self):
         expected_thread = self.thread
         if expected_thread is None: