Purified urinary follicle-stimulating hormone was used to induce ovulation in 18 patients with polycystic ovarian syndrome. Each ampule contained 75 IU of follicle-stimulating hormone and less than 0.11 IU of luteinizing hormone. Initial doses were 150 to 225 IU/day, later increased to a maximum of 375 IU, according to daily clinical controls and estradiol values. After 12 to 16 days, follicle-stimulating hormone treatment was suspended. Within 36 to 48 hours each patient received 5000 or 10,000 IU of human chorionic gonadotropin, rarely more. Ovulation occurred in 39 of 43 treatment cycles and hyperstimulation in nine. Seven patients had normal pregnancies with viable fetuses, including one pair of twins. Two had abortions. Analysis of the endocrine situation during therapy does not permit either pregnancy or hyperstimulation to be predicted. However, hyperstimulation is frequently accompanied by endogenous luteinizing hormone peaks and greater estradiol increases during the final phase of induction. Purified follicle-stimulating hormone has thus demonstrated its validity in inducing ovulation in patients with polycystic ovarian syndrome, apparently with equal or lower risks of hyperstimulation than with other gonadotropin preparations.