diff --git a/Lib/_weakrefset.py b/Lib/_weakrefset.py
--- a/Lib/_weakrefset.py
+++ b/Lib/_weakrefset.py
@@ -63,7 +63,7 @@ class WeakSet:
                     yield item
 
     def __len__(self):
-        return sum(x() is not None for x in self.data)
+        return len(self.data) - len(self._pending_removals)
 
     def __contains__(self, item):
         try:
diff --git a/Lib/test/test_weakset.py b/Lib/test/test_weakset.py
--- a/Lib/test/test_weakset.py
+++ b/Lib/test/test_weakset.py
@@ -17,6 +17,10 @@ import contextlib
 class Foo:
     pass
 
+class RefCycle:
+    def __init__(self):
+        self.cycle = self
+
 
 class TestWeakSet(unittest.TestCase):
 
@@ -359,6 +363,30 @@ class TestWeakSet(unittest.TestCase):
             s.clear()
         self.assertEqual(len(s), 0)
 
+    def test_len_race(self):
+        # Extended sanity checks for len() in the face of cyclic collection
+        self.addCleanup(gc.set_threshold, *gc.get_threshold())
+        for th in range(1, 100):
+            N = 20
+            gc.collect(0)
+            gc.set_threshold(th, th, th)
+            items = [RefCycle() for i in range(N)]
+            s = WeakSet(items)
+            del items
+            # All items will be collected at next garbage collection pass
+            it = iter(s)
+            try:
+                next(it)
+            except StopIteration:
+                pass
+            n1 = len(s)
+            del it
+            n2 = len(s)
+            self.assertGreaterEqual(n1, 0)
+            self.assertLessEqual(n1, N)
+            self.assertGreaterEqual(n2, 0)
+            self.assertLessEqual(n2, n1)
+
 
 def test_main(verbose=None):
     support.run_unittest(TestWeakSet)