Background: The impact of cardiac resynchronization therapy (CRT) on dispersion of repolarization is controversial. This study aimed to determine whether CRT alters the QT interval and Tpeak–Tend interval (Tpeak–end) and whether such changes relate to the risk of developing a major arrhythmic event (MAE). Methods: Data from 67 patients (49 men; age 71±10 years) who underwent CRT device placement were analyzed retrospectively. Patients had NYHA class III or IV heart failure. Mean left ventricular ejection fraction was 25±9%. The electrocardiogram was recorded at baseline and during follow-up after implantation (3 days, 7 days, 1 month, and 2 months). Results: After 29-months of follow-up, 11 patients had experienced MAEs. QT interval and Tpeak–end did not change significantly immediately after CRT. However, 3 days after CRT, Tpeak–end in patients with MAE was significantly increased when compared with patients without MAE (p<0.05). We divided patients into 2 groups according to the change in Tpeak–end at 3 days (increased Tpeak–end group, n=27; decreased Tpeak–end group, n=40). The increased Tpeak–end group demonstrated a significant increase in MAEs (p<0.05). Conclusions: Increased Tpeak–end at 3 days after CRT was associated with a significant increase in MAEs. This could be a useful predictor of ventricular proarrhythmia.