['To determine whether patients with congestive heart failure and true left bundle branch block (LBBB) morphology have better response to cardiac resynchronization therapy (CRT) than do patients without true LBBB.', 'We defined true LBBB as conventional LBBB plus QRS duration ≥ 130 ms and mid-QRS notching/slurring in at least two of the leads I, aVL, V1, V2, V5, or V6. We prospectively enrolled 58 patients with heart failure and allocated them to three groups: true LBBB (t-LBBB, n = 22); non-true LBBB (nt-LBBB, LBBB with no notch or notches in fewer than two of the leads, n = 17); and non-specific intraventricular conduction delay (IVCD, n = 19). At 6 month follow-up, mean absolute increases in left ventricular ejection fraction were 16.0% ± 11.6% in t-LBBB, 8.1% ± 11.2% in nt-LBBB (P = 0.02), and 3.3% ± 7.8% in IVCD (P < 0.001, t-LBBB vs. IVCD) and changes in mean New York Heart Association class were -1.2 ± 0.6 in t-LBBB, -0.8 ± 0.6 in nt-LBBB (P = 0.071), and -0.5 ± 0.6 in IVCD (P = 0.01, t-LBBB vs. IVCD). All patients with t-LBBB were responders, some were super-responders. Multivariate analysis showed that t-LBBB (odds ratio, OR, 11.680; 95% confidence interval, CI, 1.966-69.390; P = 0.007) and left ventricular end-diastolic dimension (OR, 0.891; 95% CI, 0.797-0.996; P = 0.043) are independent predictors of super-response to CRT.'] Does true complete left bundle branch block morphology strongly predict good response to cardiac resynchronization therapy?