Two agonadic sisters, one with a 46,XY and the other with a 46,XX karyotype, both with normal female external genitalia and hypoplastic Müllerian derivatives, born to a consanguineous marriage, were studied from a clinical, endocrinological, histological, and genetic perspective. Using PCR amplification, Southern hybridization, and DGGE analysis, it was found that the XY patient had no mutations in the conserved sequence of the SRY gene, the putative testis-determining gene in mammals, whereas her XX affected sister is SRY-negative. To our knowledge, this is the first report of XY and XX sibs in familial gonadal agenesis without other somatic abnormalities. The involvement of an autosomal locus impeding gonadal development in both sexes is discussed.