diff --git a/changelog.d/20124.misc b/changelog.d/20124.misc
new file mode 100644
index 0000000000..d92a12ce08
--- /dev/null
+++ b/changelog.d/20124.misc
@@ -0,0 +1 @@
+Fix cache `__len__` of Sliding Sync `PerConnectionState` ignoring account data entries.
\ No newline at end of file
diff --git a/synapse/types/handlers/sliding_sync.py b/synapse/types/handlers/sliding_sync.py
index dd913250ba..d8c90c0fdd 100644
--- a/synapse/types/handlers/sliding_sync.py
+++ b/synapse/types/handlers/sliding_sync.py
@@ -919,6 +919,7 @@ class PerConnectionState:
         receipts: The status of each room for the receipts stream.
         room_configs: Map from room_id to the `RoomSyncConfig` of all
             rooms that we have previously sent down.
+        account_data: The status of each room for the account_data stream.
     """
 
     last_used_ts: int | None = None
@@ -951,7 +952,12 @@ class PerConnectionState:
         )
 
     def __len__(self) -> int:
-        return len(self.rooms) + len(self.receipts) + len(self.room_configs)
+        return (
+            len(self.account_data)
+            + len(self.rooms)
+            + len(self.receipts)
+            + len(self.room_configs)
+        )
 
 
 @attr.s(auto_attribs=True)