The discriminatory power of a gonadotropin-releasing hormone test and a human chorionic gonadotropin test in diagnosing gonadotropin deficiency was studied in 23 prepubertal boys with hypogonadotropic hypogonadism (HH). The boys were originally referred because of genital hypoplasia, delayed sexual maturation, or suspicion of HH. The diagnosis of HH was established clinically, in most cases after follow-up of several years. The results were compared with those of a reference group consisting of 44 prepubertal boys with incomplete testicular descent. Post-hCG serum testosterone level was the most sensitive discriminating variable, and was subnormal in 11 of 12 boys with HH (in one of 16 in the reference group). Post-GnRH serum LH concentration was the second most sensitive, and was subnormal in 15 of 23 boys with HH (two of the reference group). Our data indicate that post-hCG testosterone levels are of greater value than post-GnRH gonadotropin levels in the diagnosis of HH in prepubertal boys.