The electrocardiographic and vectorcardiographic findings of 29 patients with tricuspid atresia were related to the coexistent cardiac abnormalities and the status of pulmonary blood flow. Certain electrovectorcardiographic features were found to permit distinction between those patients with tricuspid atresia who had diminished pulmonary blood flow and those who had increased flow. Lead V6 of patients with tricuspid atresia, subpulmonary stenosis and diminished pulmonary blood flow generally revealed shallow Q waves and small R waves. The vectorcardiograms of these patients revealed narrow QRS vector loops in each plane and were posteriorly related. Among the patients with tricuspid atresia without subpulmonary stenosis or with coexistent transposition of the great vessels, tall R waves, frequently in association with deep Q waves, were present in lead V6. Likewise, the QRS vector loops in each plane encompassed a large area. Most patients had left axis deviation, as anticipated. A normal axis, when present, was usually related to coexistent transposition of the great vessels. The patients with subpulmonary stenosis had right atrial enlargement in infancy but left atrial enlargement when older; those without pulmonary stenosis frequently had left or combined atrial enlargement even in infancy. The origin of the left axis deviation in these patients is uncertain. Histologic studies of the myocardium failed to reveal abnormalities that might explain the alteration.