The pubertal maturation of five boys (Group A) who were initially thought to be gonadotropin deficient was studied over 10 to 58 months (mean 36 months) by serial physical examinatons and standard GnRH tests. Four were seen because of obesity, delayed sexual maturation, depression, and poor school performance. The other boy had acquired hypothalamic hypopituitarism at 13 years of age. Gonadotropin responses during the initial GnRH test were either absent or abnormally low as related to the degree of skeletal maturation. Subsequent responses showed progressive maturation into the normal range for adult males. These boys had normal olfaction and moderate-to-marked obesity, but initial assessment of testicular size, basal gonadotropins, and testosterone or gonadotropin responses to GnRH did not distinguish these boys from seven patients with isolated gonadotropin deficiency (Group B). Contrary to previous reports and expectations, these studies indicate that an absent or markedly blunted response to synthetic GnRH is not diagnostic of gonadotropin deficiency, even when skeletal age is 12 years or greater. Furthermore, unless a patient is hyposomic or anosmic, or has an associated anomaly such as cleft palate, isolated gonadotropin deficiency cannot be diagnosed reliably until late adolescence or early adulthood.