OBJECTIVES--To compare the abortifacient efficacy and side effects of three doses of the antiprogestin mifepristone plus prostaglandin for termination of early pregnancy. DESIGN--Randomised, double blind multicentre trial. SETTING--11 departments of obstetrics and gynaecology and of family planning, mostly in university hospitals, in seven countries. SUBJECTS--1182 women with an early pregnancy (menstrual delay of 7-28 days) requesting abortion. INTERVENTIONS--Single doses of 200 mg, 400 mg, or 600 mg mifepristone followed, 48 hours later, by vaginal pessary of 1 mg of the prostaglandin E1 analogue gemeprost. MAIN OUTCOME MEASURES--Outcome of treatment; duration and subjective amount of menstrual bleeding; side effects and complications; and concentrations of haemoglobin. RESULTS--Outcome was similar with the three doses of mifepristone. Of the 1151 women with known outcome, 95.5% had a complete abortion (364 (93.8%) of those given 200 mg mifepristone, 368 (94.1%) of those given 400 mg, and 367 (94.3%) of those given 600 mg), 3.7% had an incomplete abortion (14 (3.6%), 15 (3.8%), and 14 (3.6%)), 0.3% had a missed abortion (three (0.8%), one (0.3%), and none), and 0.4% had a continuing live pregnancy (two (0.5%), two (0.5%), and one (0.3%)). Of the 43 women who had incomplete abortion, 23 underwent emergency uterine curettage (usually for haemostatic purposes) and three of these women were given a blood transfusion. The numbers of reported complaints, bleeding patterns, and changes in blood pressure and haemoglobin concentrations were similar with the three treatments. CONCLUSIONS--For termination of early pregnancy a single dose of 200 mg mifepristone is as effective as the currently recommended dose of 600 mg when used in combination with a vaginal pessary of 1 mg gemeprost.