Background: We hypothesized that preoperative electromechanical dyssynchrony amenable to cardiac resynchronization therapy (CRT) and QRS narrowing immediately after CRT are both correlated and have a cumulative impact on response and outcome after CRT. Methods: A total of 233 CRT candidates (heart failure New York Heart Association classes II-IV, ejection fraction < 35%, QRS >= 120 milliseconds, 44% women, 71 +/- 11 years old) were prospectively included. Preoperative electromechanical dyssynchrony amenable to CRT was assessed by septal deformation patterns using speckle tracking echocardiography. QRS narrowing was calculated from 12-lead electrocardiograms before and immediately after CRT implantation. The primary endpoint was overall mortality during long-term follow-up. The NTC clinical trial number is NCT02986633. Results: Eighty-seven percent of patients with preoperative electromechanical dyssynchrony experienced QRS narrowing after CRT (118/ 136), whereas 69% of patients without preoperative electromechanical dyssynchrony (67/97) experienced QRS narrowing after CRT (P < 0.001). By Cox multivariate analysis, both preoperative electromechanical dyssynchrony and lack of postoperative QRS narrowing were independently associated with an increased risk of mortality during follow-up (adjusted hazards ratio [HR] 2.24, 95% confidence interval [CI] 1.43-3.50 and HR 1.90, 95% CI 1.06-3.38, respectively). Compared with patients with preoperative electromechanical dyssynchrony, patients without both electromechanical dyssynchrony and postoperative QRS narrowing experienced a considerable increased risk of mortality during follow-up (adjusted HR 3.70, 95% CI 1.96-6.97). Conclusions: Lack of postoperative QRS narrowing after CRT is associated with preoperative electromechanical dyssynchrony. Both preoperative electromechanical dyssynchrony and postoperative QRS narrowing have a favourable cumulative impact on outcome after CRT.