The incidence of electrical instability of the heart is high in patients with absence of the right and persistence of the left superior caval vein when the latter connects to the coronary sinus. It has been suggested that a large coronary sinus may influence the susceptibility to arrhythmias. we studied the conduction tissues of 8 hearts from the cardiopathological collection of Children's Hospital of Pittsburgh. Six of these specimens had a persistent left superior caval vein connecting to the coronary sinus. There were 4 with absence of the right superior caval vein, 3 of which were in the group with persistent left veins. We evaluated the sinus node and the specialized atrioventricular junctional area, comparing them with known normals. The coronary sinus varied in size from through it. The size did not alter the histology of the adjacent conduction tissue. In contrast, the sinus node was abnormal in 3 of the 4 hearts with absent right superior caval vein. This may be the key factor in the development of arrhythmias when the right superior caval vein is absent or abnormal.