Hello. I investigated ref leak report related to thread. Please run python regrtest.py -R :: test_leak.py (attached file) Sometimes ref leak is reported. # I saw this as regression failure on python-checkins. # total ref count 92578 -> 92669 _Condition 2 Thread 6 _Event 1 bool 10 instancemethod 1 code 2 dict 9 file 1 frame 3 function 2 int 1 list 2 builtin_function_or_method 5 NoneType 2 str 27 thread.lock 7 tuple 5 type 5 Probably this happens because threading.Thread is implemented as Python code, (expecially threading.Thread#join), the code of regrtest.py if i >= nwarmup: deltas.append(sys.gettotalrefcount() - rc - 2) can run before thread really quits. (before Moudles/threadmodule.c t_bootstrap()'s Py_DECREF(boot->func); Py_DECREF(boot->args); Py_XDECREF(boot->keyw); runs) So I experimentally inserted the code to wait for thread termination. (attached file experimental.patch) And I confirmed error was gone. # Sorry for hackish patch which only runs on windows. It should run # on other platforms if you replace Sleep() in Python/sysmodule.c # sys_debug_ref_leak_leave() with appropriate function.