Starting January 1st, 2012, all new examinees will have a six-attempt limit to pass any USMLE Step or component exam (this applies if you had not made any attempt at said exam before this date). When this limit goes into effect, an examinee will be ineligible to take a Step or Step Component if the examinee has made six or more prior attempts to pass it, including incomplete attempts.
For examinees that had made any attempt, including incomplete attempts, to pass any Step or Step Component prior to January 1st, 2012, the six-attempt limit will not go into effect until January 1st, 2013. After this date, each examinee will not be allowed to apply for additional attempts at the same exam after six attempts to pass it. Prior to this date, however, an examinee may apply for additional attempts beyond his or her sixth attempt.
Please see www.usmle.org for further information.