A 15-year-old girl with pubertal masculinization due to bilateral gonadoblastoma and 45,X/46,X,+mar karyotype.

We report the case of a 15-year-old girl with bilateral gonadoblastoma and 45,X/46,X,+mar karyotype. She was short but had no other stigmata of Turner syndrome. Her genitalia were completely of the female type until she began to show signs of masculinization at age 13 years. She had breasts of Tanner stage 4, primary amenorrhea, clitoromegaly, hirsutism and low voice. Plasma testosterone level was elevated and returned to normal after the successful removal of the bilateral gonadoblastoma. Her karyotype was 45,X/46,X,+mar and it was proved that the marker chromosome was derived from the short arm of the Y chromosome by Southern blot analysis with a Y chromosome specific probe, pDP1007. Even when there are no stigmata of Turner syndrome or signs of intrauterine virilization, the possibility of gonadoblastoma should not be ruled out in girls who show postnatal virilization.