The purpose of this study was to determine whether basal or stimulated (or both) serum follicle-stimulating hormone (FSH) and luteinizing hormone (LH) on day 3 of the cycle before administration of exogenous gonadotropins can predict stimulation response and in vitro fertilization (IVF) outcome. Eighty consecutive new patients underwent a gonadotropin-releasing hormone (GnRH) stimulation test on the morning of cycle day 3. All patients underwent the same stimulation protocol consisting of a combination of FSH and human menopausal gonadotropin (hMG). Paired discriminant analysis of FSH0 (at 0 minutes from GnRH injection) and LH0 revealed seven distinct groups of patients with statistically significant differences among the means: groups 1, 2, and 3 (26.25%) with higher means FSH0:LH0; group 4 (40%) with mean FSH0:LH0 (both levels less than 10 mIU/ml) of 1:1, and groups 5, 6, and 7 (33.75%) with higher mean LH0:FSH0. Canonical discriminant analysis of both basal and stimulated serum FSH and LH levels confirmed the seven groups and did not add to the information from analysis of FSH0 and LH0 only. Serum estradiol (E2) response during stimulation, as well as the number of preovulatory oocytes aspirated and transferred, was highest in the groups with a higher mean LH0:FSH0, intermediate in the group with mean FSH0:LH0 of 1:1, and lowest in the group with a higher mean FSH0:LH0. No pregnancy occurred in the higher FSH:LH groups. It is concluded that basal serum gonadotropin levels can distinguish different populations of IVF patients who tend to behave differently in terms of E2 response, oocytes obtained and transferred, and pregnancy rates and outcome.