OBJECTIVES To compare the efficacy of two different regimens of mifepristone followed by misoprostol for medical abortion in women with menstrual delay of < or = 35 days. DESIGN Double-blind, randomised controlled trial. SETTING Seventeen centres internationally. PARTICIPANTS We enrolled 1,589 healthy pregnant women with menstrual delay of < or = 35 days who were requesting nonsurgical abortion. INTERVENTIONS Within gestational age strata, we randomly assigned women to receive a single oral dose of mifepristone, either 200 mg or 600 mg, followed in 48 h by misoprostol 400 microg by mouth. We concealed the allocation assignments from investigators and participants and maintained double-blinding throughout the study. MAIN OUTCOME MEASURES Complete abortion was the principal outcome measure. We also compared rates of side effects such as abdominal pain. RESULTS The complete abortion rate with the lower dose of mifepristone was similar to that with the higher dose (89.3% vs 88.1%) The crude relative risk of failure to achieve complete abortion with the 200 mg dose compared with the 600 mg dose was 0.9 (95% CI 0.7 to 1.2). The likelihood of complete abortion was inversely related to gestational age, although this finding is exploratory in nature. Among failures the percentage of women with continuing pregnancies increased from 1.4% at menstrual delay of two weeks or less to 9.0% when the delay was 4-5 weeks. Low efficacy led to stopping enrolment at 29 to 35 days' menstrual delay. Stopping criteria were also met at completion of the study in the group with 22-28 days' menstrual delay. No significant differences emerged in the frequency of side effects between the two mifepristone groups. CONCLUSIONS Both regimens had similar efficacy. Women with a menstrual delay of four to five weeks had twice the risk of failure to abort compared with those who received treatment within two weeks of the expected menses. The efficacy of the mifepristone-prostaglandin regimen was not reduced by decreasing the dose of mifepristone from 600 mg to 200 mg. The regimens of 600 mg or 200 mg of mifepristone, followed by a single oral dose of misoprostol 400 microg 48 hours later, were not sufficiently efficient in inducing abortion when the menstrual delay was > 21 days.