• The discriminative power of the gonadotropin releasing hormone test and the human chorionic gonadotropin (HCG) test in the diagnosis of gonadotropin deficiency was studied in 73 boys referred because of delayed pubertal development or suspicion of gonadotropin deficiency. Hypogonadotropic hypogonadism was confirmed by clinical follow-up in 21 of the boys and excluded in the others because of normal pubertal development. Those latter boys served as a reference group. The post-HCG serum testosterone level was subnormal in hypogonadotropic hypogonadism on 12 of 19 occasions (in the reference group on two of 46 occasions) and the post–gonadotropin releasing hormone serum luteinizing hormone level was subnormal on fourteen of 22 occasions (zero of 65). Four of the seven boys with hypogonadotropic hypogonadism who had normal post-HCG testosterone levels had subnormal peak luteinizing hormone levels. Of the remaining three boys, two had low basal testosterone levels. Combining the two tests therefore improved the diagnostic accuracy.