diff -r 8fe59931b64d Lib/concurrent/futures/_base.py
--- a/Lib/concurrent/futures/_base.py	Thu Jan 23 11:25:48 2014 +0100
+++ b/Lib/concurrent/futures/_base.py	Thu Jan 23 14:39:29 2014 +0100
@@ -190,16 +190,18 @@ def as_completed(fs, timeout=None):
     if timeout is not None:
         end_time = timeout + time.time()
 
-    with _AcquireFutures(fs):
-        finished = set(
-                f for f in fs
-                if f._state in [CANCELLED_AND_NOTIFIED, FINISHED])
-        pending = set(fs) - finished
-        waiter = _create_and_install_waiters(fs, _AS_COMPLETED)
+    waiter = _AsCompletedWaiter()
+    pending = set()
+    for f in fs:
+        with f._condition:
+            if f._state in [CANCELLED_AND_NOTIFIED, FINISHED]:
+                yield f
+            else:
+                pending.add(f)
+                f._waiters.append(waiter)
 
+    installed_waiters = frozenset(pending)
     try:
-        yield from finished
-
         while pending:
             if timeout is None:
                 wait_timeout = None
@@ -222,8 +224,9 @@ def as_completed(fs, timeout=None):
                 pending.remove(future)
 
     finally:
-        for f in fs:
-            f._waiters.remove(waiter)
+        for f in installed_waiters:
+            with f._condition:
+                f._waiters.remove(waiter)
 
 DoneAndNotDoneFutures = collections.namedtuple(
         'DoneAndNotDoneFutures', 'done not_done')