A 28-year-old male pseudohermaphrodite with gynaecomastia was raised as a female until the age of 17 years, at which time he developed masculine features (deepening of the voice, development of facial hair, male distribution of body hair and male body habitus) and assumed a male gender role. He had a small phallus with perineal urethra, absence of labioscrotal fusion, presence of vaginal pouch and undescended testes. The testicular biopsy showed hyalinization of the tubular basement membrane, lack of spermatogenesis and hyperplastic Leydig cells. Baseline peripheral plasma studies showed androstenedione concentrations ten times normal, low testosterone, elevated oestrone and elevated gonadotrophins. The in vitro incubation of testicular tissue showed no significant conversion of androstenedione to testosterone. However, two types of peripheral tissues, skin fibroblasts and erythrocytes, had a normal conversion, as did the body overall as measured by the technique of androstenedione constant infusion. These studies demonstrate that the 17-ketosteroid reductase deficiency of the patient was limited to the testes.