T-Wave Predicts CRT Response in LBBB Patients Introduction Chronic heart failure patients with a left ventricular (LV) conduction delay, mostly due to left bundle branch block (LBBB), generally derive benefit from cardiac resynchronization therapy (CRT). However, 30–50% of patients do not show a clear response to CRT. We investigated whether T-wave analysis of the ECG can improve patient selection. Methods and Results The study population comprised 244 CRT recipients with baseline 12-lead electrocardiogram recordings. Echocardiographic response after 6-month CRT was defined as a ≥5% increase in LV ejection fraction (LVEF). Vectorcardiograms (VCGs) were constructed from the measured 12-lead ECGs using an adapted Kors algorithm on digitized ECGs. Logistic regression models indicated repolarization variables as good predictors of CRT response. The VCG-derived T-wave area predicted CRT response (odds ratio [OR] per 10 μVs increase 1.172 [P < 0.001]) even better than QRS-wave area (OR = 1.116 [P = 0.001]). T-wave area had especially predictive value in the LBBB patient group (OR = 2.77 in LBBB vs. 1.09 in non-LBBB). This predictive value persisted after adjustment of multiple covariates, such as gender, ischemia, age, hypertension, coronary artery bypass graft, and the usage of diuretics and β-blockers. In LBBB patients, the increase in LVEF was 6.1 ± 9.7% and 11.3 ± 9.1% in patients with T-wave area below and above the median value, respectively (P < 0.01). Conclusion In patients with LBBB morphology of the QRS complex, a larger baseline T-wave area is an important independent predictor of LVEF increase following CRT.