The objectives of this study were to determine the effectiveness, side effects, and acceptability of one-third the standard 600 mg dose of mifepristone (200 mg) to induce abortion. A prospective trial at seven sites enrolled women > or = 18 years, up to 8 weeks pregnant, and wanting an abortion. The women received 200 mg mifepristone orally, self-administered 800 micrograms misoprostol vaginally at home 48 h later, and returned 1-4 days later for ultrasound evaluation. Surgical intervention was indicated for continuing pregnancy, excessive bleeding, persistent products of conception 5 weeks later, or other serious medical conditions. Of the 933 subjects, 906 (97%) had complete medical abortions, 22 had surgical intervention, two were protocol failures, and three were lost to follow up. Of the 746 subjects who had no or minimal bleeding before misoprostol, 80% bled within 4 h and 98% within 24 h of using misoprostol. By day 7, 95% of women had a complete abortion. Side effects were aceptable in 85% of subjects, and 94% found the procedure acceptable. Low-dose mifepristone followed by vaginal misoprostol was highly effective as an abortifacient.