Communication of the coronary sinus with the left atrium is an unusual anomaly. Two cases of tricuspid atresia are described where systemic desaturation developed following a modified Fontan procedure. In both, investigation demonstrated a communication between the coronary sinus and the left atrium. Transcatheter closure of these communications was achieved with a ductal device in one case and coil embolization in the other. Following occlusion and elimination of the right-to-left shunt, both patients became fully saturated in room air.