Left bundle branch block (LBBB) is the most reliable electrocardiographic predictor of responsiveness to cardiac resynchronization therapy (CRT). However, not all patients with LBBB will respond to CRT. Our aim was to investigate the interaction between QRS duration, LBBB-type morphology, and the responsiveness to CRT. We retrospectively analyzed electrocardiograms of 243 patients who underwent CRT implantation according to current clinical indications. A 6-month reduction of left ventricular end-systolic volume >15% was used to identify CRT responders. The clinical end point consisted of death, hospitalization for heart failure and sustained rapid ventricular tachyarrhythmias. An LBBB morphology was present in 169 patients (70%) and 101 of these (60%) were responders to CRT. Analyzing the interaction between QRS duration and CRT responsiveness in patients with LBBB, a “U shaped” distribution resulted, with nonresponders clustered between 120 and 130 ms and above 180 ms. The receiver operating characteristic curve analysis identified 178 ms as the optimal cut-off value of QRS to predict a nonresponsiveness to CRT (area under the curve = 0.67 [95% confidence interval 0.57 to 0.76]). At multivariate analysis, only an ischemic cause and a QRS ≥178 ms were independent predictors of nonresponsiveness to CRT (area under the curve = 0.75). Patients with LBBB with QRS ≥178 ms had greater likelihood of adverse clinical events during a mean follow-up of 32 months (p = 0.049). In conclusion, in patients with LBBB undergoing CRT, a marked QRS widening (i.e., ≥178 ms) is related to worse echocardiographic responsiveness and lower event free survival rate compared with patients with an intermediate QRS widening.