OBJECTIVE: To assess the efficacy and safety of a vaginal progesterone (P(4)) insert (Endometrin) for luteal support for assisted reproductive technology (ART). DESIGN: Multicenter, randomized, open-label (assessor-blinded) phase III clinical trial. SETTING: Twenty-five U.S. ART centers. PATIENT(S): A total of 1,211 ART patients randomized to three groups: Endometrin 100 mg twice daily (n = 404), Endometrin 100 mg three times daily (n = 404), and P(4) 90 mg 8% gel daily (n = 403). INTERVENTION(S): In vitro fertilization and ET were performed according to site-specific protocols. The day after oocyte retrieval, Endometrin or vaginal P(4) gel was begun for luteal support and continued for up to 10 weeks of pregnancy. MAIN OUTCOME MEASURE(S): Biochemical, clinical, and ongoing pregnancy and live birth rates. RESULT(S): Pregnancy rates were high and similar in all treatment groups, with biochemical rates exceeding 50%, clinical and ongoing rates >or=40%, and live birth rates at 35%-38%. The adverse event profiles were similar across groups. CONCLUSION(S): Pregnancy rates and live birth rates for Endometrin (twice daily and three times daily) were high and similar to those for P(4) gel. The adverse event profiles for both were similar to that for P(4) gel and primarily due to IVF stimulation and oocyte retrieval. Endometrin was safe and well tolerated.