Twenty cases of cor triatriatum are reported. The diagnosis was confirmed by necropsy in 16 cases and at the time of operation in 4. The lesion occurred as an isolated anomaly in 7 cases; in 13, other associated cardiac anomalies were present. Three anatomic types of cor triatriatum were identified in the cases studied at necropsy: diaphragmatic (10 cases), hourglass (3) and tubular (3). The diaphragmatic type was also present in all four cases in which the diagnosis was confirmed at operation. Associated anomalies were found in five cases of the diaphragmatic type and in each case of the hourglass of tubular types. In isolated cor triatriatum the clinical findings were characteristic of pulmonary venous and arterial hypertension. In two cases, one with a communication between the right atrium and the accessory left atrial chamber and one with partial anomalous pulmonary venous connection associated with cor triatriatum, the clinical findings suggested a large left to right shunt with pulmonary arterial hypertension. The clinical findings varied in the cases with associated anomalies, and it was difficult to determine the cause of disturbance of the circulation.