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Eight cases of termination of the left superior vena cava in the left atrium are reviewed pathologically and clinically. In three of the five patients who died, pathologic examination revealed three anomalies which, together, are considered to form a developmental complex. The anomalies are (1) termination of the left superior vena cava in the left atrium, (2) absence of the coronary sinus, and (3) an atrial septal defect lying in the postero-inferior angle of the atrial septum. In the two remaining fatal cases, the aforementioned anomalies were associated with persistent common atrioventricular canal. In this situation, the atrial septal defect of the latter malformation was confluent with the atrial septal defect of the anomalous complex described. The coexistence of three anomalies-the left superior vena cava terminating in the left atrium, absent coronary sinus, and atrial septal defect-is considered to result from a single developmental abnormality. This takes the form of failure of complete formation of the left atriovenous fold, that fold which normally develops along the left side of the junction of the sinus venosus and the atrial portion of the heart. Clinically, features of increased pulmonary blood flow, coupled with duskiness of the skin, were the significant abnormalities. When cardiac catheterization was attempted through a right-sided vein, the data revealed a left-to-right shunt at atrial level, while levels of arterial oxygen desaturation were present. Except in cases with coexistent ventricular septal defect, pulmonary hypertension was absent. In each of the four patients in whom cardiac catheterization was performed through a left antecubital vein, the catheter was advanced into the left superior vena cava, left atrium, and left ventricle. This was substantiated by angiocardiography in three of the four patients. In our experience, angiocardiography was the most reliable procedure to substantiate the termination of the left superior vena cava in the left atrium. The present study suggests that, in the absence of pulmonary hypertension, a left-to-right transatrial shunt associated with systemic arterial oxygen desaturation may indicate the presence of a persistent left superior vena cava terminating in the left atrium. Moreover, an atrial septal defect identified surgically as lying in the postero-inferior angle of the atrial septum should suggest that an additional anomaly may be present in the form of termination of the left superior vena cava in the left atrium.