Two 46 XY siblings with familial incomplete male pseudohermaphroditism, Type 2, inherited as an apparent autosomal recessive trait, were investigated. The phenotype was distinctive in that male wolffian-duct structures (epididymis, vas deferens and seminal vesicles) were present whereas tissues derived from the urogenital sinus and from the anlage of the external genitalia were female in character. Studies of estrogen and androgen dynamics revealed normal male blood testosterone and estrogen levels, production rates and interconversions. The normal blood luteinizing hormone level and normal estrogen production rate suggest that androgen resistance in this syndrome is different from that in other androgen-resistant states. The demonstration of markedly deficient dihydrotestosterone formation in slices of perineal skin, epididymis and phallus, considered with the fact that dihydrotestosterone is the fetal hormone responsible for male differentiation of the external genitalia, is compatible with the possibility that the fundamental lesion is defective formation of dihydrotestosterone in these genital tissues. (N Engl J Med 291:944–949, 1974)