Anti-Müllerian hormone (AMH), alias Müllerian-inhibiting substance or factor, plays a key role in fetal sex differentiation. The cloning of the human gene, a member of the transforming growth-factor-beta family and the development of immunochemical reagents recognizing circulating human AMH have opened new perspectives for clinical research. AMH assays and genetic studies now provide meaningful information regarding testicular function in infancy and the molecular basis of a rare form of male pseudohermaphroditism, the persistent Müllerian duct syndrome.