Successful correction of cor triatiatum in a 3-month-old infant is presented. The diagnosis was established preoperatively by separate angiocardiographic visualization of the two left atrial compartments. To our knowledge all successful total corrections were done under cardiopulmonary bypass. In this case, the abnormal diaphragm in the left atrium was resected under normothermic inflow occlusion. We believe that the technically very simple resection of the diaphragm does not necessitate the use of extracorporeal circulation in an infant, provided the diagnosis can be made beyond any doubt before the operation.