Although most asymptomatic patients with the Wolff-Parkinson-White electrocardiographic pattern have a good prognosis, some die suddenly. The mechanism of sudden death is usually ventricular fibrillation, which is triggered by atrial fibrillation with a rapid ventricular response rate. Electrophysiologic testing has been proposed to identify asymptomatic patients who may be at risk for sudden death. Meaningful application of such testing requires a knowledge of whether the electrophysiologic measurements are reproducible over time. Consequently, we performed electrophysiologic studies on two occasions at least 36 months apart (mean +/- SD, 54.7 +/- 14) in 29 asymptomatic patients with the pattern. Twenty-seven patients remained asymptomatic, and sustained supraventricular tachycardia developed in two during the follow-up period. Nine patients (31 percent) lost the capacity for preexcitation and anterograde conduction over the accessory pathway, which produces the Wolff-Parkinson-White pattern. The others had little change in measurements of conduction over the accessory pathway. Patients who lost conduction over the accessory pathway tended to be older (mean +/- SD, 50 +/- 18 vs. 39 +/- 11 years; P = 0.06) than patients who retained preexcitation, and they had longer anterograde effective refractory periods at the first assessment (414 +/- 158 vs. 295 +/- 27 msec; P = 0.003). We conclude that a considerable number of asymptomatic patients with the Wolff-Parkinson-White pattern lose their capacity for anterograde conduction over the accessory pathway. This loss of capacity probably contributes to the low mortality among asymptomatic patients.