diff --git a/Lib/timeit.py b/Lib/timeit.py
--- a/Lib/timeit.py
+++ b/Lib/timeit.py
@@ -328,6 +328,10 @@
         else:
             sec = msec / 1000
             print("best of %d: %.*g sec per loop" % (repeat, precision, sec))
+    worst = max(r)
+    if worst >= best * 4.0:
+        print "eWarning, these test results likely aren't reliable.  The worst"
+        print "time was more than four times slower than the best time."
     return None
 
 if __name__ == "__main__":