One of two brothers with the virilizing adrenogenital syndrome manifested slow psychomotor development and was initially diagnosed as having “infantile autism”. He developed an osteogenic sarcoma, and skin, lymphocytes and cultured tumor cells showed a 47, XYY karyotype. The younger brother shows normal behavior and has a 46, XY chromosome constitution. Both children showed the same hormonal response to steroid suppression therapy, but the XYY boy continued in an abnormal growth pattern.