Patients with polycystic ovarian disease were grouped into three groups according to their maximum ovarian diameter (maxD) after ovulation induction by purified urinary follicle-stimulating hormone (FSH). Serum luteinizing hormone (LH) and FSH were measured daily by radioimmunoassay and size and number of follicles were assessed by ultrasonography. Follicle-stimulating hormone in group A (80 mm less than or equal to maxD) was significantly higher than those of group B (60 mm less than or equal to maxD less than 80 mm) and C (maxD less than 60 mm) for the last 4 days of the treatment. This FSH rise in group A was not accounted for by FSH accumulation by the study of pharmacodynamics, and so was thought to be of endogenous origin. Luteinizing hormone was also elevated 3 days before the administration of human chorionic gonadotropin (hCG) in both groups A and B. The number of follicles in group A at hCG administration was significantly greater than that of group C. Any significant differences were not found in either total amount of purified urinary FSH or in the basal FSH and the LH levels before treatment of the three groups. These results suggest that excessive ovarian enlargement during gonadotropin therapy is caused by multiple follicular development primarily stimulated by endogenous FSH. Endogenous LH release further enhances ovarian enlargement.