Division of the left atrium is a rare congenital lesion, but one which is amenable to complete surgical correction. We describe twin brothers, demonstrated to be monozygotic, who presented in infancy with cardiac failure, pulmonary congestion and mild cyanosis. Both were found to have systemic and pulmonary venous return to a common atrial chamber, separated from the left atrial appendage and the vestibule supporting the mitral valve by a typical muscular diaphragm with a single small orifice. In one twin, there was also persistence of the left superior caval vein, while, in the other twin, a remnant of the primary atrial septum was present. Excision of the diaphragm and neoseptation of the atrial chambers was performed in both infants. At 10 years after surgery, there was normal growth and development in both twins, with no evidence of recurrent obstruction.