OBJECTIVES: To evaluate the efficacy of combined mifepristone and misoprostol compared to misoprostol alone in outpatient medical treatment of first trimester miscarriage. Additionally, the study intends to compare the rate of complications, adverse effects, and treatment acceptability between groups. STUDY DESIGN: Single-center double-blind randomized placebo-controlled trial including women with diagnosis of missed first trimester miscarriage up to 9 weeks of gestation. RESULTS: Between April 2019 and November 2021, 216 women diagnosed with first trimester miscarriage up to 9 weeks of gestation were randomly assigned to mifepristone group or to misoprostol-alone group. Data from 105 women in mifepristone group and 103 women in misoprostol-alone group were analyzed, with no differences in baseline characteristics. The median time between medications (oral mifepristone/placebo and vaginal misoprostol) was nearly 43 h in both groups (p = 0.906). The median time to first follow-up was 2.6 weeks (IQR 1.0) in mifepristone group and 2.4 weeks (IQR 1.0) in misoprostol-alone group (p = 0.855). The overall success rate of medical treatment was significantly higher in the mifepristone-group comparing to misoprostol-alone group (94.3% vs. 82.5%, RR 1.14, 95% CI, 1.03-1.26; p = 0.008). Accordingly, the rate of surgical treatment was significantly lower in the mifepristone-group (5.7% vs.14.6%, RR 0.39, 95% CI, 0.16-0.97; p = 0.034). The composite complication rate was similar and lower than 4% in both groups. No case of complicated pelvic infection, hemodynamic instability or inpatient supportive treatment was reported. There were no significant differences in the rates of adverse events, median score for vaginal bleeding intensity or analgesics use. Despite the same median value, the score of abdominal pain intensity was significantly higher in the mifepristone-group (p = 0.011). In both groups, more than 65% of the women classified the treatment as "good" and 92% would recommend it to a friend on the same clinical situation. CONCLUSION: The mifepristone plus vaginal misoprostol combined treatment for medical resolution of first trimester miscarriage resulted in significant higher success rate and lower rate of surgical uterine evacuation comparing to misoprostol-alone treatment, with no relevant differences in adverse events or treatment acceptability.