diff --git a/synapse/storage/client_ips.py b/synapse/storage/client_ips.py
index 747d2df62..014ab635b 100644
--- a/synapse/storage/client_ips.py
+++ b/synapse/storage/client_ips.py
@@ -20,6 +20,8 @@ from twisted.internet import defer
 from ._base import Cache
 from . import background_updates
 
+import os
+
 logger = logging.getLogger(__name__)
 
 # Number of msec of granularity to store the user IP 'last seen' time. Smaller
@@ -28,12 +30,15 @@ logger = logging.getLogger(__name__)
 LAST_SEEN_GRANULARITY = 120 * 1000
 
 
+CACHE_SIZE_FACTOR = float(os.environ.get("SYNAPSE_CACHE_FACTOR", 0.1))
+
+
 class ClientIpStore(background_updates.BackgroundUpdateStore):
     def __init__(self, hs):
         self.client_ip_last_seen = Cache(
             name="client_ip_last_seen",
             keylen=4,
-            max_entries=5000,
+            max_entries=50000 * CACHE_SIZE_FACTOR,
         )
 
         super(ClientIpStore, self).__init__(hs)