Testicular function was evaluated in forty-one prepubertal patients with male pseudohermaphroditism by determining serum concentrations of progesterone, 17-hydroxyprogesterone, dehydroepiandrosterone, androstenedione, testosterone and dihydrotestosterone before and after stimulation with hCG and, in some instances, ACTH. Testosterone response to hCG was normal in all subjects. In one patient, a 4-year-old boy, a deficiency of 17,20-desmolase activity was diagnosed based on the coexistence of elevated levels of pregnenolone, 17-hydroxypregnenolone, progesterone and 17-hydroxyprogesterone and low levels of dehydroepiandrosterone and androstenedione. In three other patients enzymatic blocks were suspected but not confirmed. Congenital deficiency of enzymes necessary for testosterone biosynthesis is an uncommon aetiology of male pseudohermaphroditism.