We studied the effects of the progesterone antagonist RU 486 in 100 women with early, unwanted pregnancy (within 10 days of the expected onset of the missed menstrual period). Thirty-four women received oral doses of 400 mg (in four days), 26 received 600 mg (in four days), and 40 received 800 mg (in two days). Uterine bleeding occurred in all patients within four days of the first dose and continued for 5 to 17 days. In 85 of the women, a dramatic decrease in the plasma chorionic gonadotropin level was observed on day 6, and an empty uterus was confirmed by ultrasonography on day 13. Hence, these women were considered to have had a complete abortion. Fifteen subjects had persistently elevated plasma chorionic gonadotropin levels on day 6 and were considered not to have responded to RU 486. They all had uterine evacuation, which was facilitated by a softening of the cervix. The percentage of women with complete abortion was similar in all dosage groups. Furthermore, plasma levels of immunoreactive RU 486 were similar in subjects with and without complete abortion. The only important side effect observed in the responders was prolonged uterine bleeding in 18 percent, but neither blood transfusion nor curettage was required. We conclude that RU 486 is an effective and safe method for termination of very early pregnancy but that it should be used only under close medical supervision.