ABSTRACT A family is presented in which all males of the second generation died at birth, and in which two out of four individuals of the third generation exhibited ambiguous external genitalia. All members of the third generation, one brother and one sister in one sibship, and two brothers in another sibship, were studied from both a clinical and a cytogenetical point of view. In the two boys with ambiguous external genitalia the legal sex was designated female at birth but was later changed to male. Histological examination of their gonads revealed testicular dysgenesis. The girl was clinically normal, and the third male had a small penis and cryptorchid testes. All the boys had an apparently normal 46/XY karyotype without any evidence of mosaicism. It could not be decided from studies on the Xg blood group and some other sex-linked characters whether the condition described was inherited as an X-linked recessive or an autosomal dominant, sex-limited trait.