We have previously shown that prophylactic supplementation of progesterone beginning in the luteal phase of patients treated with human menopausal gonadotropins (hMG) could reduce the risk of spontaneous abortions. The present study was initiated with 100 patients to evaluate the efficacy of a new progesterone therapeutic regime in patients requiring either hMG or clomiphene citrate. A significantly decreased risk of spontaneous abortion (6% vs. 28%) was seen in 50 patients prophylactically treated with progesterone as compared with 50 control patients. The progesterone regimen was then tried on 566 consecutive patients who were treated and conceived with hMG or clomiphene citrate, and approximately the same risk (6.2% by 20 weeks) was found. This incidence of spontaneous abortion is even less than the accepted risk for the general population.