Ten (10) sets of siblings or parent–child pairs concordant for gender identity disorder (transsexualism) or gender identity disorder and transvestitism are reported. For concordant gender identity disorder, there is one set of male monozygotic twins; three sets of non-twin brothers; one brother-and-sister pair; one set of sisters; and one father and son. With gender identity disorder and transvestism, there is one transsexual father with a gender dysphoric; transvestic son; one transvestic father with a gender dysphoric, transvestic son; and one transvestic father with a transsexual daughter. The emerging technology of genetic markers makes collation of such families a potentially valuable resource for unraveling the origins of atypical gender identity.