Analysis of the clinical findings and growth in 20 boys with isolated gonadotropin deficiency revealed a heterogeneous group of physical abnormalities. Ten of these patients were hyposmic or anosmic (Kallmann syndrome). Abnormalities found in our patients included undescended testes, gynecomastia, and ocular or skeletal anomalies. Regardless of the presence of hyposmia, patients without testicular enlargement (less than 2 cm3), had serum luteinizing hormone (LH) responses to luteinizing hormone-releasing factor (LRF) that were the same as in prepubertal boys. By contrast, five boys with testicular enlargement (greater than 2 cm3), some of whom had hyposmia, had a greater serum LH response to LRF than did prepubertal boys. Adrenarche was moderately delayed; although all boys initially had normal serum levels of dehydroepiandrosterone-sulfate, four boys eventually developed elevated serum levels. Bone ages were delayed compared with chronologic age in boys who had the condition after 15 years of age. The rate of linear growth was normal, and final adult heights were normal with testosterone therapy, although linear growth continued longer in these boys than in boys with normal pubertal progression. Although none of the patients was obese at the time of diagnosis, three patients developed obesity after initiation of testosterone therapy.