Caudal regression syndrome is a rare congenital condition characterized by varying degrees of developmental failure ranging from a partial sacral agenesis to the absence of lumbosarcal spine, hypoplasia, or fusion of the lower extremities and visceral anomalies. This is the third case of only one of the twins involved by this syndrome described in the literature and the second case of the selective involvement in dizygotic twins. Selective involvement of only one twin suggests that factors other than hyperglycemia and 7q deletions may be involved in the pathogenesis.