PIP: This article presents the results of a study comparing the trials of two different regimens of emergency contraceptive pills (ECPs) in the US. A 21-center multinational randomized controlled trial compared the Yuzpe ECP regimen of combined oral contraceptives with a progestin-only regimen using levonorgestrel. Findings indicated that in both study groups, the efficacy of both treatments declined with increasing time following unprotected intercourse. Furthermore, delaying the first dose by 12 hours increases the odds of pregnancy by about 50%. This finding supports the earlier finding from investigators in New Zealand, who reported that Yuzpe regimen is more effective if taken within 12 hours than within 13-24 hours. This finding has two clinical implications: advance provision of a prescription; need for clinicians to develop some mechanisms for providing ECPs on weekends. The results of this study, particularly on the timing of levonorgestrel-only ECP are further investigated in a large multicenter trial including 2800 women using progestin-only regimen by the year 2000.