A patient with 46,XY karyotype and 17a-hydroxylase deficiency is reported who illustrates marked virilization of the external genitalia. Marked phallic development and almost complete labioscrotal fusion with no development of a utriculovaginal pouch were noted. Because the perineum was essentially similar to that seen in male-to-female transsexuals, vaginal construction required the Mclndoe procedure. Hence, although some patients with 17α -hydroxylase deficiency have minimal virilization of the external genitalia, this patient's history indicates the operative management'necessary in the opposite extreme, when patients show complete masculinization. Hormonal evaluation after gonadectomy revealed the enzyme deficiency based on abnormalities of steroid secretion by the adrenal cortex.