diff -r 6343bdbb7085 Lib/subprocess.py
--- a/Lib/subprocess.py	Mon Feb 10 03:43:57 2014 -0800
+++ b/Lib/subprocess.py	Mon Feb 10 15:44:35 2014 +0200
@@ -461,22 +461,20 @@
 except:
     MAXFD = 256
 
-# This lists holds Popen instances for which the underlying process had not
-# exited at the time its __del__ method got called: those processes are wait()ed
-# for synchronously from _cleanup() when a new Popen object is created, to avoid
-# zombie processes.
-_active = []
+# This lists holds PIDs of processes which had not exited at the time
+# corresponding Popen instance's __del__ method got called: those processes
+# are wait()ed for synchronously from _cleanup() when a new Popen object is
+# created, to avoid zombie processes.
+_active = set()
 
-def _cleanup():
-    for inst in _active[:]:
-        res = inst._internal_poll(_deadstate=sys.maxsize)
-        if res is not None:
-            try:
-                _active.remove(inst)
-            except ValueError:
-                # This can happen if two threads create a new Popen instance.
-                # It's harmless that it was already removed, so ignore.
-                pass
+def _cleanup(_active=_active, _waitpid=os.waitpid, _WNOHANG=os.WNOHANG):
+    for pid in _active.copy():
+        try:
+            if _waitpid(pid, _WNOHANG)[0] != pid:
+                continue
+        except OSError:
+            pass
+        _active.discard(pid)
 
 PIPE = -1
 STDOUT = -2
@@ -890,18 +888,20 @@
         # Wait for the process to terminate, to avoid zombies.
         self.wait()
 
-    def __del__(self, _maxsize=sys.maxsize, _active=_active):
-        # If __init__ hasn't had a chance to execute (e.g. if it
-        # was passed an undeclared keyword argument), we don't
-        # have a _child_created attribute at all.
-        if not getattr(self, '_child_created', False):
-            # We didn't get to successfully create a child process.
-            return
-        # In case the child hasn't been waited on, check if it's done.
-        self._internal_poll(_deadstate=_maxsize)
-        if self.returncode is None and _active is not None:
-            # Child is still running, keep us alive until we can wait on it.
-            _active.append(self)
+    if not mswindows:
+        def __del__(self, _maxsize=sys.maxsize):
+            # If __init__ hasn't had a chance to execute (e.g. if it
+            # was passed an undeclared keyword argument), we don't
+            # have a _child_created attribute at all.
+            if not getattr(self, '_child_created', False):
+                # We didn't get to successfully create a child process.
+                return
+            # In case the child hasn't been waited on, check if it's done.
+            self._internal_poll(_deadstate=_maxsize)
+            active = _active
+            if self.returncode is None and active is not None:
+                # Child is still running, keep us alive until we can wait on it.
+                active.add(self.pid)
 
     def _get_devnull(self):
         if not hasattr(self, '_devnull'):
diff -r 6343bdbb7085 Lib/test/test_subprocess.py
--- a/Lib/test/test_subprocess.py	Mon Feb 10 03:43:57 2014 -0800
+++ b/Lib/test/test_subprocess.py	Mon Feb 10 15:44:35 2014 +0200
@@ -54,8 +54,9 @@
         support.reap_children()
 
     def tearDown(self):
-        for inst in subprocess._active:
-            inst.wait()
+        if not mswindows:
+            for pid in subprocess._active:
+                os.waitpid(pid, 0)
         subprocess._cleanup()
         self.assertFalse(subprocess._active, "subprocess._active not empty")
 
@@ -1982,6 +1983,7 @@
         finally:
             p.wait()
 
+    @unittest.skipIf(mswindows, "Posix specific test")
     def test_zombie_fast_process_del(self):
         # Issue #12650: on Unix, if Popen.__del__() was called before the
         # process exited, it wouldn't be added to subprocess._active, and would
@@ -1994,12 +1996,12 @@
                              stderr=subprocess.PIPE)
         self.addCleanup(p.stdout.close)
         self.addCleanup(p.stderr.close)
-        ident = id(p)
         pid = p.pid
         del p
-        # check that p is in the active processes list
-        self.assertIn(ident, [id(o) for o in subprocess._active])
+        # check that pid is in the active processes list
+        self.assertIn(pid, subprocess._active)
 
+    @unittest.skipIf(mswindows, "Posix specific test")
     def test_leak_fast_process_del_killed(self):
         # Issue #12650: on Unix, if Popen.__del__() was called before the
         # process exited, and the process got killed by a signal, it would never
@@ -2013,24 +2015,23 @@
                              stderr=subprocess.PIPE)
         self.addCleanup(p.stdout.close)
         self.addCleanup(p.stderr.close)
-        ident = id(p)
         pid = p.pid
         del p
         os.kill(pid, signal.SIGKILL)
         # check that p is in the active processes list
-        self.assertIn(ident, [id(o) for o in subprocess._active])
+        self.assertIn(pid, subprocess._active)
 
         # let some time for the process to exit, and create a new Popen: this
-        # should trigger the wait() of p
+        # should trigger the waitpid() of pid
         time.sleep(0.2)
         with self.assertRaises(OSError) as c:
             with subprocess.Popen(['nonexisting_i_hope'],
                                   stdout=subprocess.PIPE,
                                   stderr=subprocess.PIPE) as proc:
                 pass
-        # p should have been wait()ed on, and removed from the _active list
+        # pid should have been waitpid()ed on, and removed from the _active list
         self.assertRaises(OSError, os.waitpid, pid, 0)
-        self.assertNotIn(ident, [id(o) for o in subprocess._active])
+        self.assertNotIn(pid, subprocess._active)
 
     def test_close_fds_after_preexec(self):
         fd_status = support.findfile("fd_status.py", subdir="subprocessdata")