46,XX maleness and 46,XX 21-hydroxylase deficiency in dizygotic twins: association or coincidence?

46,XX maleness is a rare abnormality of gonadal differentiation. We present dizygotic twins, one having ambiguous genitalia due to 2-hydroxylase deficiency, and the other having normal male genitalia with 46,XX maleness. One of the twins was referred to the endocrinology unit at 2 days old because of ambiguous genitalia. The other twin with bilateral undescended testes located in the inguinal canal was diagnosed with 46,XX maleness. The karyotype was 46,XX. Molecular analysis revealed the presence of SRY in the latter twin without Müllerian structures. Association of congenital adrenal hyperplasia (46,XX 21-hydroxylase deficiency) and 46,XX maleness in twins has not been previously reported.