Atrioventricular re-entrant tachycardia is the most common form of tachycardia in patients with Wolff'Parkinson'White (WPW) syndrome where one or more accessory pathways are present between atria and ventricles. Paroxysmal atrial fibrillation (PAF) is also a relatively common form of tachycardia in these patients. The mechanisms of PAF in patients with WPW syndrome are not completely understood. Previous studies have mainly focused on the role of accessory pathways in the development of PAF. However, up to 24% of patients still experience PAF after successful ablation of accessory pathways by surgical incision or radiofrequency catheter ablation techniques. We hypothesized that there are two primary mechanisms involved in the pathogenesis of PAF in patients with WPW syndrome. One is related to the accessory pathways that predispose the atria to fibrillation; the other is increased atrial vulnerability that is independent of the accessory pathways. Clarification of these mechanisms is critical in developing more effective means for the prevention or treatment of PAF in patients with WPW syndrome.