Clinical and electrophysiologic studies were performed in 10 aged cases (50-82 years of age) with WPW syndrome and following results were obtained: 1) Episodes of paroxysm of supraventricular tachyarrhythmias were observed in 7 and complication of sinus node dysfunction was noted in 3. 2) P-delta intervals on electrocardiogram exceeding 0.12 sec were observed in 4 cases. 3) Patterns of normalization of QRS complex in 4 cases were characterized by tachycardia-dependent in 2 and bradycardia-dependent in 2. 4) Effective refractory period of the accessory pathway (AP) in antegrade direction exceeded that of A-V node in 5 cases and the former exceeding 500 msec was observed in 2 cases. The relations between the changes of the electrophysiologic properties of the AP and altered clinical manifestations of the WPW syndrome in aged cases were discussed.