This study was performed to compare the efficacy of Crinone 8% intravaginal gel and intramuscular progesterone (IMP) supplementation for luteal phase support in women under age 40 undergoing IVF-ET. Results of the interim analysis of this rigorously designed prospective randomized controlled study are presented. Pregnancy rates, implantation rates, and early spontaneous abortion rates were similar for patients receiving Crinone or IMP, whereas fewer side effects and greater overall satisfaction were reported by women receiving Crinone.