Coronary sinus (CS) ostial obstruction decompressing through a left superior vena cava is a rare finding and presents unique risks to patients requiring a Glenn anastomosis for single-ventricle palliation. Preoperative diagnosis is imperative in order to avoid inadvertent ligation or coil occlusion. We report three cases with single-ventricle physiology and CS obstruction identified at cardiac catheterization. Balloon occlusion angiography proved valuable in making the diagnosis.