['To examine the effects of subtle elevation in P levels in late follicular phase on the outcome of IVF-ET cycles, using GnRH agonist (GnRH-a) and hMG +/- FSH protocol.', 'A retrospective analysis of data.', 'Fifty-four patients who completed 63 IVF-ET cycles were treated with midluteal GnRH-a, followed by hMG +/- pure FSH. Depending on serum P levels on the day of hCG administration, patients were divided in two groups. In group 1, P levels were < or = 0.9 ng/mL (conversion factor to SI unit, 3.180) and in group 2, the levels were > 0.9 ng/mL.', 'Luteinizing hormone levels, on the day of hCG administration, as measured by RIA, were suppressed completely. In cycles with subtle P rise (71%), we observed a significantly higher serum E2 concentration, greater number of mature follicles, and greater number of oocytes retrieved. There were no differences between the two groups in fertilization rate, number of embryos transferred, clinical pregnancy rate, implantation rate, and miscarriage or delivery rates.'] Do elevated progesterone levels in the late follicular phase predict success of in vitro fertilization-embryo transfer?