There are reports associating an increased incidence of Wolff-Parkinson-White syndrome with tricuspid atresia. Here we report on electrophysiologic studies in patients with tricuspid atresia and Wolff-Parkinson-White syndrome after the Fontan operation. In these patients the atrial arrhythmia often seen in patients undergoing the Fontan operation triggered atrioventricular re-entrant tachycardia or caused life-threatening arrhythmias.