Twelve patients with cor triatriatum sinistrum were treated over a 28-year period. Their ages ranged from 1 month to 7.5 years. Congestive heart failure was the most common presentation. Cardiac catheterization was performed on six of the 12 patients and a correct diagnosis of cor triatriatum was made on angiography in only four of the six. Of the remaining six patients, three were diagnosed as having cor triatriatum by echocardiography and three by autopsy. Echocardiography is now considered to be the diagnostic modality of choice in our institution. Seven patients were operated on and five died prior to diagnosis or treatment Associated cardiac anomalies included persistent left superior vena cava, atrial septal defects, coarctation of the aorta, and total anomalous pulmonary venous drainage. A right atrial, transseptal approach to the common pulmonary chamber and excision of the left atrial membrane was found to be the treatment of choice and was used in six of the seven patients operated on. One patient died in the postoperative period. Thus, cor triatriatum sinistrum, a rare and potentially lethal congenital cardiac anomaly, can be diagnosed by echocardiography and successfully treated surgically with a low operative mortality.