To evaluate the existence of a peculiar atrial electrophysiologic substrate, we studied 18 patients with asymptomatic Wolff-Parkinson-White (WPW) syndrome. These patients were compared with 10 age-matched normal subjects (N). Effective and functional refractory periods were determined at two right atrial sites (high and low in the lateral wall), during atrial pacing (100 min-1) and at twice diastolic threshold. Disperson (D) of effective (ERP) and functional (FRP) refractoriness was evaluated as the difference between refractory periods at the two atrial sites. WPW patients showed significantly lower mean values of effective and functional refractoriness at both atrial sites and significantly higher mean values of D-ERP and D-FRP. Moreover, in calculating the highest normal values of D-ERP and D-FRP (as mean values of N plus 2SD) it was observed that WPW with abnormal values of D showed a statistically (chi 2 test) higher incidence (100%) of induced atrial fibrillation (AF). These findings indicate the existence of both an abnormal atrial electrophysiologic substrate and of a higher vulnerability in WPW. Finally, AF was induced generally at the site with the lower refractoriness (i.e. low lateral site). This should be taken into account when considering how atrial fibrillation can be induced more easily.