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Abnormal early pregnancy is a common experience for women. Approximately 1 in 4 women will experience a miscarriage during her lifetime.1 Between 15% and 20% of clinically recognized pregnancies are diagnosed as abnormal in the first or early second trimester.2 For more than 50 years, the standard management of early pregnancy failure has been a dilation and curettage (D&C). Typically, the procedure is performed in an operating room, which significantly increases cost. In recent years, the medical community began to question whether immediate evacuation by surgical intervention is truly necessary for most cases of early pregnancy failure.3–5 Misoprostol, a prostaglandin E1 analogue, has garnered attention over the past few decades, both in regimens for medical abortion and by itself for early pregnancy failure.6 Overall, comparison of trials using misoprostol for early pregnancy failure is difficult because the studies included various patient populations and dosing regimens (for example, repeat dosing every 3–4 hours or every 24 hours), different routes of administration (oral or vaginal), varying definitions of success (the time allowed until dilation and curettage was performed),7 and a misunderstanding of how to use ultrasonography to define both inclusion and success.8–10 Additionally, most of the large studies involve inpatient admission for medical management, a scenario that would be unlikely in the United States.11,12 To address these weaknesses, we conducted a multicenter randomized trial including 491 women treated with misoprostol 800 μg vaginally and 161 women who had a vacuum aspiration procedure for early pregnancy failure.13 Misoprostol treatment success was 71% (95% confidence interval [CI] 67–75%) after one dose and 84% (95% CI 81–87%) overall. Although this overall success was statistically equivalent to the 97% (95% CI 94–100%) for vacuum aspiration, the rates are clinically quite different. There may be easily identifiable subgroups for whom misoprostol treatment will be more or less effective and, ideally, some women for whom the success rates may more closely approximate those of vacuum aspiration. This report details the findings of a planned substudy of this multicenter trial to evaluate factors related to success with misoprostol treatment.