Index: Lib/concurrent/futures/_base.py
===================================================================
--- Lib/concurrent/futures/_base.py	(revision 86476)
+++ Lib/concurrent/futures/_base.py	(working copy)
@@ -12,6 +12,7 @@
 FIRST_COMPLETED = 'FIRST_COMPLETED'
 FIRST_EXCEPTION = 'FIRST_EXCEPTION'
 ALL_COMPLETED = 'ALL_COMPLETED'
+_AS_COMPLETED = '_AS_COMPLETED'
 
 # Possible future states (for internal use by the futures package).
 PENDING = 'PENDING'
@@ -70,8 +71,30 @@
     def add_cancelled(self, future):
         self.finished_futures.append(future)
 
+class _AsCompletedWaiter(_Waiter):
+    """Used by as_completed()."""
+
+    def __init__(self):
+        super(_AsCompletedWaiter, self).__init__()
+        self.lock = threading.Lock()
+
+    def add_result(self, future):
+        with self.lock:
+            super(_AsCompletedWaiter, self).add_result(future)
+            self.event.set()
+
+    def add_exception(self, future):
+        with self.lock:
+            super(_AsCompletedWaiter, self).add_exception(future)
+            self.event.set()
+
+    def add_cancelled(self, future):
+        with self.lock:
+            super(_AsCompletedWaiter, self).add_cancelled(future)
+            self.event.set()
+
 class _FirstCompletedWaiter(_Waiter):
-    """Used by wait(return_when=FIRST_COMPLETED) and as_completed()."""
+    """Used by wait(return_when=FIRST_COMPLETED)."""
 
     def add_result(self, future):
         super().add_result(future)
@@ -128,7 +151,9 @@
             future._condition.release()
 
 def _create_and_install_waiters(fs, return_when):
-    if return_when == FIRST_COMPLETED:
+    if return_when == _AS_COMPLETED:
+        waiter = _AsCompletedWaiter()
+    elif return_when == FIRST_COMPLETED:
         waiter = _FirstCompletedWaiter()
     else:
         pending_count = sum(
@@ -187,11 +212,15 @@
                             '%d (of %d) futures unfinished' % (
                             len(pending), len(fs)))
 
-            waiter.event.wait(timeout)
+            waiter.event.wait(wait_timeout)
 
-            for future in waiter.finished_futures[:]:
+            with waiter.lock:
+                finished = waiter.finished_futures
+                waiter.finished_futures = []
+                waiter.event.clear()
+
+            for future in finished:
                 yield future
-                waiter.finished_futures.remove(future)
                 pending.remove(future)
 
     finally: