Patients: A 46,XY mother who developed as a normal woman underwent spontaneous puberty, reached menarche, menstruated regularly, experienced two unassisted pregnancies, and gave birth to a 46,XY daughter with complete gonadal dysgenesis. Results: Evaluation of the Y chromosome in the daughter and both parents revealed that the daughter inherited her Y chromosome from her father. An extensive family pedigree across four generations revealed multiple other family members with ambiguous genitalia and infertility in both phenotypic males and females, and the mode of inheritance of the phenotype was strongly suggestive of X-linkage. Conclusions: The range of phenotypes observed in this unique family suggests that there may be transmission of a mutation in a novel sex-determining gene or in a gene that predisposes to chromosomal mosaicism.