Background; The coexistence of two distinct atrioventricular (AV) nodes has been described in complex congenital heart disease requiring Fontan type palliation. The purpose of this study was to evaluate the occurrence of twin AV node according to anatomical subgroups, and to determine its relation to tachycardia. Methods; From 2001 to 2003, we performed an electrophysiologic study upon 52 consecutive patients who had undergone cardiac catheterization after Fontan completion. Atrial pacing was performed at 3 or more different atrial sites. Results; In 10/52 patients, two different QRS complexes were recorded at different pacing sites, suggesting twin AV node (9/20 in heterotaxy syndrome, 1/8 AV discordance, 0/24 other complex anomalies). AV reciprocating tachycardia (AVRT), presumably involving two AV nodes and a connecting sling, was induced in 6 of 10 patients who had twin AV node (4/6 used posterior AV node as an antegrade limb, 2/6 used an anterior AV node as an antegrade limb). Heterotaxy syndrome (p<0.001) and common atrioventricular canal (p=0.002) were found to be risk factors for twin AV node. Junctional tachycardia (JT; HR>150/min) with either VA dissociation (7/9) or second degree VA block (2/9) were induced by pacing or isoproterenol infusion in 9/52 patients. Junctional tachycardia induction was associated with a twin AV node (p=0.04), or a history of early postoperative junctional ectopic tachycardia (p=0.02). Conclusion; A complicated AV node conduction system such as twin AV node was frequent in heterotaxy syndrome. Both AVRT, and JT with VA block may be important causes of tachyarrhythmia in this patient group.