The triatrial heart (cor triatriatum) is one of the most fascinating of the cardiac anomalies. For over a century anatomists have debated the nature of the embryological defect. At present most descriptions refer to one type of anomaly, the so-called "stenosis of the common pulmonary vein." The very persistence of disagreement on the pathogenesis of the triatrial heart would lead one to believe there are several anomalies involved.In addition to stenosis of the common pulmonary vein several other types of triatrial hearts have been described, often only as anatomic curiosities. In order to clarify our concepts of hearts with three supraventricular chambers, it is considered worthwhile to review the reported anomalies and attempt a logical classification.To understand the pathologic development of triatrial hearts it is essential to become familiar with the embryology of the pulmonary veins, the left superior vena cava and the interatrial septum. It is not the purpose of this report to review this information which is available elsewhere. Particularly useful are the studies of Patten ('31, '38, '53), the classic monograph of Marshall (1850), and selected discussions of normal and anomalous pulmonary veins (Benda, '24; Grantigan, '47;