diff --git a/Lib/sched.py b/Lib/sched.py
--- a/Lib/sched.py
+++ b/Lib/sched.py
@@ -34,6 +34,9 @@
 __all__ = ["scheduler"]
 
 class Event(namedtuple('Event', 'time, priority, action, argument')):
+    def __init__(self, *args, **kwargs):
+        super(Event, self).__init__(*args, **kwargs)
+        self._cancelled = False
     def __eq__(s, o): return (s.time, s.priority) == (o.time, o.priority)
     def __ne__(s, o): return (s.time, s.priority) != (o.time, o.priority)
     def __lt__(s, o): return (s.time, s.priority) <  (o.time, o.priority)
@@ -42,10 +45,13 @@
     def __ge__(s, o): return (s.time, s.priority) >= (o.time, o.priority)
 
 class scheduler:
+    cancellations_threshold = 1024
+
     def __init__(self, timefunc, delayfunc):
         """Initialize a new instance, passing the time and delay
         functions"""
         self._queue = []
+        self._cancellations = 0
         self.timefunc = timefunc
         self.delayfunc = delayfunc
 
@@ -76,8 +82,8 @@
         If the event is not in the queue, this raises ValueError.
 
         """
-        self._queue.remove(event)
-        heapq.heapify(self._queue)
+        self._cancellations += 1
+        event._cancelled = True
 
     def empty(self):
         """Check whether the queue is empty."""
@@ -114,14 +120,21 @@
             time, priority, action, argument = checked_event = q[0]
             now = timefunc()
             if now < time:
+                if self._cancellations > self.cancellations_threshold:
+                    self._cancellations = 0
+                    self._queue = [x for x in self._queue if not x._cancelled]
+                    heapq.heapify(self._queue)
                 delayfunc(time - now)
             else:
                 event = pop(q)
                 # Verify that the event was not removed or altered
                 # by another thread after we last looked at q[0].
                 if event is checked_event:
-                    action(*argument)
-                    delayfunc(0)   # Let other threads run
+                    if event._cancelled:
+                        self._cancellations -= 1
+                    else:
+                        action(*argument)
+                        delayfunc(0)   # Let other threads run
                 else:
                     heapq.heappush(q, event)