diff --git a/changelog.d/5774.misc b/changelog.d/5774.misc
new file mode 100644
index 000000000..5e15dfd5f
--- /dev/null
+++ b/changelog.d/5774.misc
@@ -0,0 +1 @@
+Reduce database IO usage by optimising queries for current membership.
diff --git a/synapse/storage/roommember.py b/synapse/storage/roommember.py
index a4936022e..e60409ed7 100644
--- a/synapse/storage/roommember.py
+++ b/synapse/storage/roommember.py
@@ -156,9 +156,12 @@ class RoomMemberWorkerStore(EventsWorkerStore):
             # then we can avoid a join, which is a Very Good Thing given how
             # frequently this function gets called.
             if self._current_state_events_membership_up_to_date:
+                # Note, rejected events will have a null membership field, so
+                # we we manually filter them out.
                 sql = """
                     SELECT count(*), membership FROM current_state_events
                     WHERE type = 'm.room.member' AND room_id = ?
+                        AND membership IS NOT NULL
                     GROUP BY membership
                 """
             else:
@@ -180,10 +183,13 @@ class RoomMemberWorkerStore(EventsWorkerStore):
             # we order by membership and then fairly arbitrarily by event_id so
             # heroes are consistent
             if self._current_state_events_membership_up_to_date:
+                # Note, rejected events will have a null membership field, so
+                # we we manually filter them out.
                 sql = """
                     SELECT state_key, membership, event_id
                     FROM current_state_events
                     WHERE type = 'm.room.member' AND room_id = ?
+                        AND membership IS NOT NULL
                     ORDER BY
                         CASE membership WHEN ? THEN 1 WHEN ? THEN 2 ELSE 3 END ASC,
                         event_id ASC