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 14:28:52 2014 +0100
@@ -155,8 +155,10 @@ def _create_and_install_waiters(fs, retu
     elif return_when == FIRST_COMPLETED:
         waiter = _FirstCompletedWaiter()
     else:
-        pending_count = sum(
-                f._state not in [CANCELLED_AND_NOTIFIED, FINISHED] for f in fs)
+        with _AcquireFutures(fs):
+            pending_count = sum(
+                    f._state not in [CANCELLED_AND_NOTIFIED, FINISHED]
+                    for f in fs)
 
         if return_when == FIRST_EXCEPTION:
             waiter = _AllCompletedWaiter(pending_count, stop_on_exception=True)
@@ -165,8 +167,9 @@ def _create_and_install_waiters(fs, retu
         else:
             raise ValueError("Invalid return condition: %r" % return_when)
 
-    for f in fs:
-        f._waiters.append(waiter)
+    with _AcquireFutures(fs):
+        for f in fs:
+            f._waiters.append(waiter)
 
     return waiter
 
@@ -222,8 +225,9 @@ def as_completed(fs, timeout=None):
                 pending.remove(future)
 
     finally:
-        for f in fs:
-            f._waiters.remove(waiter)
+        with _AcquireFutures(fs):
+            for f in fs:
+                f._waiters.remove(waiter)
 
 DoneAndNotDoneFutures = collections.namedtuple(
         'DoneAndNotDoneFutures', 'done not_done')
@@ -269,8 +273,9 @@ def wait(fs, timeout=None, return_when=A
         waiter = _create_and_install_waiters(fs, return_when)
 
     waiter.event.wait(timeout)
-    for f in fs:
-        f._waiters.remove(waiter)
+    with _AcquireFutures(fs):
+        for f in fs:
+            f._waiters.remove(waiter)
 
     done.update(waiter.finished_futures)
     return DoneAndNotDoneFutures(done, set(fs) - done)