Cor triatriatum is a rare congenital malformation of the heart in which a diaphragm stretches in a transverse plane through the left pulmonary venous chamber, thus creates two subchambers. The proximal chamber connects with the pulmonary veins, and the distal one has left auricle and the mitral valve. A 3 year old boy who had Cor triatriatum underwent surgical excision of the abnormal diaphragm in March, 1989 in Chungnam University Hospital. An obliquely oriented fibromuscular diaphragm divided the left atrium into a proximal chamber which was connected to the pulmonary veins and a distal chamber which had the atrial appendage and the mitral valve. The opening in the diaphragm was 5 mm in diameter. There were no associated abnormalities. The abnormal diaphragm was completely excised. The postoperative result was excellent.