Partially unroofed coronary sinus syndrome with persistent left superior vena cava is a rare congenital anomaly. It frequently produces a right-to-left shunt. We report a case in which a large left-to-right shunt was present, because of the obstruction to left ventricular inflow as a result of the dilated coronary sinus. This abnormality was associated with absent right superior vena cava and a partial pericardial defect on the right side.