Two siblings with 46,XY male pseudohermapthroditism were demonstrated to have the phenotype characteristic of 5 alpha-reductase deficiency, namely normal testes and male Wolffian duct derivatives (epididymis, vas deferens, and seminal vesicle) terminating in a blind-ending vagina. Clitoromegaly was present at birth and increased further at the time of expected puberty. The diagnosis of 5 alpha-reductase deficiency was confirmed by demonstration of male levels of testosterone and testosterone precursors before and after hCG administration, elevated plasma testosterone to dihydrotestosterone and urinary etiocholanolone to androsterone ratios, and by in vitro studies indicating 5 alpha-reductase enzyme deficiency in the epididymis of one patient. Studies of control and mutant epididymal microsomes indicated that a single enzyme is responsible in the normal person for the 5 alpha-reduction of testosterone and cortisol (and probably other delta 4-3-ketosteroids as well) and that 5 alpha-reductase activity is undetectable for all substrates examined in the mutant. This finding explains why the formation of 5 alpha-reduced glucocorticoids is also defective in the disorder.