BACKGROUND: The dose of mifepristone approved by most government agencies for medical abortion is 600 mg. Our aim was to summarize extant data on the effectiveness and safety of regimens using the widely recommended lower mifepristone dose, 200 mg, followed by misoprostol in early pregnancy and to explore potential correlates of abortion failure. STUDY DESIGN: To identify eligible reports, we searched Medline, reviewed reference lists of published reports, and contacted experts to identify all prospective trials of any design of medical abortion using 200 mg mifepristone followed by misoprostol in women with viable pregnancies up to 63 days' gestation. Two authors independently extracted data from each study. We used logistic regression models to explore associations between 15 characteristics of the trial groups and, separately, the rates of medical abortion failure and of ongoing pregnancy. RESULTS: We identified 87 trials that collectively included 120 groups of women treated with a regimen of interest. Of the 47,283 treated subjects in these groups, abortion outcome data were reported for 45,528 (96%). Treatment failure occurred in 2,192 (4.8%) of these evaluable subjects. Ongoing pregnancy was reported in 1.1% (499/45,150) of the evaluable subjects in the 117 trial groups reporting this outcome. The risk of medical abortion failure was higher among trial groups in which at least 25% of subjects had gestational age >8 weeks, the specified interval between mifepristone and misoprostol was less than 24 h, the total misoprostol dose was 400 mcg (rather than higher), or the misoprostol was administered by the oral route (rather than by vaginal, buccal, or sublingual routes). Across all trials, 119 evaluable subjects (0.3%) were hospitalized, and 45 (0.1%) received blood transfusions. CONCLUSIONS: Early medical abortion with mifepristone 200 mg followed by misoprostol is highly effective and safe.