Basically it means that you have exhausted every available thread and PHP can not start a new one. I recommend running PHP as CGI, application/x-httpd-php “/php/php.exe”, instead of using php5_module “/php/sapi/php5apache2.dll” to solve that problem. As a side note the win32 version of Apache can only handle a maximum pool of 600 threads, anything over that the threads get held in a wait proccess, no matter what ThreadLimit or Threadsperchild is set to. Yes the thread get assigned, but it not executed until the current working thread limit falls back below 600! Apache running under Win(64) or running under any version of Linux does not have those limitations.