To determine the role of the accessory pathway in the pathogenesis of atrial fibrillation, we compared electrophysiological findings in 17 patients (44.7±10.2 years) with a history of atrial fibrillation before and after surgical ablation of the accessory pathway. The PA interval was shortened, and the atrial refractory periods and the potential minimal wavelength of an atrial impulse (FRP_A/PA) were significantly increased, after surgery. Fragmented atrial activity (an increase of 150% or more in the duration of the high right atrial electrogram) was observed in 80% of the patients before surgery and in 25% after surgery. Its zone was significantly decreased after surgery. Repetitive atrial firing was defined as the occurrence of 3 or more successive atrial electrograms induced by a premature stimulation. This was observed in 60% of the patients before surgery, but in none after surgery. Atrial fibrillation was induced in 16 patients during the preoperative study, but in only 1 patient postoperatively. In conclusion, these results suggest that accessory pathways affect atrial vulnerability and play an important role in the onset of atrial fibrillation in WPW syndrome.