We studied a 35-year-old patient with female external genitalia, primary amenorrhea and XY karytotype. Plasma testosterone was 10 ng per deciliter, which did not change after administration of human chorionic gonadotropin, increased to 22 ng per deciliter after ACTH, and decreased to 0.9 ng per deciliter after dexamethasone. Plasma delta 4-androstenedione, dehydroepiandrosterone and 17-hydroxyprogesterone were in the normal range. Plasma luteinizing hormone was high, but follicle-stimulating hormone normal (7.5 mlU per milliliter). There were two testes with epididymis and vas deferens, but no Mullerian structures. Microscopical examination showed hyalinization of tubules, which were lined by normal Sertoli cells and occasional immature germ cells. No Leydig cells were seen. After castration follicle-stimulating hormone increased to 43 mlU per milliliter. We conclude that this case of male pseudohermaphroditism was probably due to a Leydig-cell agenesis, that the epididymis and vas deferens can be developed in such a condition and the follicle-stimulating hormone secretion is regulated, at least in part, by a non-androgen substance secreted by Sertoli cells.