Cycle day 3 basal levels of follicle-stimulating hormone (FSH), luteinizing hormone (LH), and estradiol (E2) were measured in 441 patients in 758 consecutive cycles to determine their predictive value for stimulation quality and pregnancy rates in vitro fertilization (IVF). Patients with low basal FSH levels (less than 15 mIU/ml) had higher pregnancy rates per attempt than those with moderate levels (15 to 24.9 mIU/ml), both of which were higher than those with high FSH levels (greater than 25 mIU/ml). Basal LH and E2 values did not improve the predictive value beyond that provided by FSH. Ongoing pregnancy rates per attempt in the low, moderate, and high FSH groups were 17.0%, 9.3%, and 3.6%, respectively (P less than 0.01). The three groups differed significantly in the percentage of patients having two ovaries, the mean number of follicles aspirated per retrieval, the mean number of preovulatory oocytes obtained, and peak E2 values (P less than 0.01). Cycle day 3 FSH levels are predictive of pregnancy outcome and stimulation characteristics in IVF, and may be useful in counseling patients.