OBJECTIVE: To compare the abortifacient efficacy and side-effects of 200 mg and 600 mg of mifepristone, followed by gemeprost 1 mg vaginally, at 57 to 63 days' gestation. DESIGN: Double-blind, randomized controlled trial. SETTING: Ten international centers. SUBJECTS: Eight hundred and ninety-six healthy women requesting medical abortion. INTERVENTIONS: Participants were randomly assigned to receive a single oral dose of mifepristone of either 200 mg or 600 mg followed in 48 h by gemeprost 1 mg vaginally. The allocation sequence was concealed by using a central pharmacy, and double masking was maintained throughout the study. MAIN OUTCOME MEASURES: Complete abortion rate was the principal outcome measure. We also evaluated the incidence of side-effects and time to abort. RESULTS: The complete abortion rate with the lower dose of mifepristone was similar to that with the higher dose (92.4% vs. 91.7%). The relative risk of failure to achieve a complete abortion with the 200 mg dose compared to 600 mg dose was 0.9 (95% CI 0.6-1.4). The timing of the abortion and the incidence of side-effects were comparable in both groups, with the exception of reported nausea at one-week follow-up which was reported more frequently by women in the higher-dose group. CONCLUSIONS: The 200 mg dose of mifepristone is equally as effective as the 600 mg dose in the antiprogestogen-prostaglandin regimen for pregnancy termination. With vaginal gemeprost, the abortifacient efficacy of the regimen remains high at 57-63 days' gestation.