A 7-month-old child with normal male external genitalia and normal, descended testicles had a 45,X karyotype in blood, skin, and testis cultures. He was mentally retarded and had a variety of bone alterations indicating disturbances of growth and ossifications. Dermatoglyphic patterns have several of the characteristics found in the 45,X female subjects with Turner's syndrome. This seems to be the first case of an "XO male" with normal external genitalia. We postulate the involvement of a "sex reversal" gene as in the origin of the XX males, with a secondary, coincidental loss of one X chromosome.