Treatment with low-dose follicle-stimulating hormone (FSH) is associated with a high rate of ovulation in anovulatory women with polycystic ovarian syndrome (PCOS), but it is not clear whether the success of treatment is because of the use of pure FSH or the low dose of gonadotropin. We undertook a randomized controlled study to compare the effects of urinary FSH and human menopausal gonadotropin (hMG) using a low-dose regimen in 30 women with PCOS. Each subject received a maximum of three cycles of either FSH or hMG. Ovulation occurred in 75% of subjects and in 77% of cycles induced with FSH and in 94% of women, 85% of cycles of those treated with hMG. A single dominant follicle developed in 70% (FSH) and 65% (hMG) of cycles, respectively. Five singleton pregnancies occurred in each group. This study shows that low-dose FSH and hMG are equally successful in inducing ovulation, suggesting that the success of treatment depends on the low dose of gonadotropin used rather than the presence or absence of luteinizing hormone in the preparation.