Background In patients with Brugada syndrome (BS), ventricular fibrillation (VF) occurs mainly during sleep; therefore, not only vagal activity but also bradycardia dependent changes in ECG may relate to the nighttime occurrence of VF. The present study aimed to examine the difference in bradycardia-dependent changes in the ECG of symptomatic and asymptomatic BS patients. Methods and Results Twenty-one patients with BS were categorized into symptomatic (n=9) and asymptomatic (n=12) groups. During the electrophysiologic study, the ECG changes were evaluated at RR intervals of 400, 600, 750, 1,000 and 1,100 ms during extrastimulation from the right atrium. The ST levels in V₂, and the QT interval in both V₂ and V₅ were measured. Along with an increase in the RR interval from 400 to 1,100 ms, the ST levels in V₂ increased in both groups; the increase did not differ between the 2 groups. In both leads V₂ and V₅, the prolongation of the QT interval along with an increase in the RR interval from 400 to 1,100 ms was significantly smaller and the QT intervals at an RR interval of 1,100 ms were significantly shorter in the symptomatic than in the asymptomatic group. Conclusions In patients with BS, the ST elevation was augmented during bradycardia to a similar extent in both symptomatic and asymptomatic patients. However, a inhibited prolongation of the QT interval during bradycardia was characteristic of symptomatic patients. These unique repolarization dynamics could relate to the nighttime occurrence of VF during bradycardia in patients with BS. (Circ J 2006; 70: 896 - 901)