From May 1960 to January 1992, 13 patients with cor triatriatum underwent surgical correction at the Mayo Clinic. Their ages ranged from 7 months to 57 years. Four patients had isolated cor triatriatum; three others had an associated patent foramen ovale or atrial septal defect. Six patients had major associated cardiac anomalies. In patients who had isolated cor triatriatum or associated anomalies with high pulmonary blood flow, the mean gradient across the left atrial diaphragm was 24.4 mm Hg, in comparison with 11.6 mm Hg in patients who had cor triatriatum and a connection between the common pulmonary venous chamber and the right atrium or associated cardiac anomalies that restricted pulmonary blood flow. In all 11 patients who underwent angiography, echocardiography, or both at our institution, the correct diagnosis was established preoperatively. Currently, echocardiography is the procedure of choice for diagnosing cor triatriatum. The diaphragm was excised through a left atrial approach in seven patients and through a right atriotomy in six. All associated anomalies were corrected. One critically ill patient who underwent an emergency operation in 1963 died early postoperatively, and one patient with chromosomal abnormalities and multiple cardiac defects died 2 months after an uneventful postoperative course. At the time of follow-up of the 11 long-term survivors, 9 were in New York Heart Association functional class I, and 2 (both of whom had major associated cardiac anomalies) were in class II. Postoperative angiography or echocardiography (in nine patients) showed no residual interatrial shunt or recurrent left atrial diaphragm.