['To determine if the level of serum P drawn on the day of hCG administration predicts assisted reproductive technology (ART) outcome in patients undergoing stimulation with hMG under GnRH agonist (GnRH-a) suppression.', 'Retrospective P assay of stored serum.', 'One hundred seventy-one patients (189 cycles) who had undergone GnRH-a suppression (leuprolide acetate or nafarelin) and stimulation with hMG for an ART procedure.', 'Progesterone RIA of serum obtained on the day of hCG administration. Measurement of sequential serum LH values by RIA in those patients with the highest P levels.', 'Pregnancy rates per oocyte retrieval were not correlated with the P level before hCG administration. There were 18 of 54 (33.3%) clinical pregnancies in those cycles with P < 0.9 ng/mL (conversion factor to SI unit, 3.180) and 42 of 135 (31.1%) clinical pregnancies in cycles with a P > or = 0.9 ng/mL. Significantly higher serum E2 levels and numbers of of follicles and oocytes obtained were observed in the high P cycles. There were no differences in the number of oocytes fertilized, the number of embryos transferred, or the implantation rate. However, a significantly higher percentage of mature oocytes were fertilized in the low P cycles (73%), as compared with the high P cycles (60%).'] Do elevated serum progesterone levels on the day of human chorionic gonadotropin administration predict outcome in assisted reproduction cycles?