A patient is described presenting with atrial fibrillation. A dilated coronary sinus was found due to the presence of a persistent left superior caval vein. Absence of the right superior caval vein was suspected with contrast injection through an i.v. line in the right arm, and was confirmed with phlebography. No associated cardiac anomalies were found. Persistent left superior vena cava is a common anomaly, although simultaneous complete absence of the right superior vena cava is rare. The incidence, embryology, diagnosis and importance of this anomaly is discussed.