In 2115 women seeking voluntary termination of pregnancy after 49 days of amenorrhea or less, we studied the effect of a single 600-mg dose of mifepristone (RU 486), followed 36 to 48 hours later by the administration of one of two prostaglandin analogues, either gemeprost (1 mg by vaginal suppository) or sulprostone (0.25, 0.375, or 0.5 mg by intramuscular injection). The women were monitored for four hours after prostaglandin administration. Efficacy was indicated by the complete expulsion of the conceptus without the need of an additional procedure. All other results were considered failures, and the pregnancy was then terminated by a surgical method. The overall efficacy rate was 96.0 percent (95 percent confidence interval, 95.0 to 96.8). The failures included persisting pregnancies (1.0 percent), incomplete expulsions (2.1 percent), and the need for hemostatic procedure (0.9 percent). The mean time to expulsion was significantly shorter when sulprostone was given in the high dose (4.5 hours) than when it was given in the two lower doses (13.1 and 19.3 hours) or when gemeprost was given (22.7 hours). The mean duration of uterine bleeding was 8.9 days (range, 1 to 35); one woman received a blood transfusion. Most women had transient abdominal pain after receiving prostaglandin, but there were few other side effects. We conclude that the administration of mifepristone followed by a small dose of a prostaglandin analogue is an effective and safe method for the early termination of pregnancy.