We evaluated the efficacy of gonadotropin treatment in stimulating spermatogenesis in men with hypogonadotropic hypogonadism. When 21 men with hypogonadotropic hypogonadism were treated with human chorionic gonadotropin, the sperm count increased to within the normal range in the 6 in whom hypogonadism had begun after puberty, but in only 1 of the 15 in whom it had begun before puberty (P less than 0.002). When the remaining 14 men with prepubertal hypogonadism were treated with human menopausal gonadotropin in addition to human chorionic gonadotropin, the sperm count increased to normal in 5 of the 7 who had not had cryptorchidism, but in only 1 of the 7 who had (P less than 0.05). The need for human menopausal gonadotropin as a replacement for follicle-stimulating hormone could not be predicted by pretreatment serum and urinary levels of follicle-stimulating hormone. We conclude that gonadotropin treatment will usually increase the sperm count to normal in men with hypogonadotropic hypogonadism, unless cryptorchidism has occurred. The need for human menopausal gonadotropin treatment appears to depend on the time of onset of hypogonadism.