OPEN ACCESSThis open-access article is distributed under the terms of the Creative Commons Attribution Non-Commercial License (CC BY-NC) (http:// creativecommons.org/licenses/by-nc/4.0/), which permits reuse, distribution and reproduction of the article, provided that the original work is properly cited and the reuse is restricted to noncommercial purposes. AIM:The aim was to evaluate clinical, morphological, functional features and mortality in patients with congestive heart failure (CHF) and different time of the best response to cardiac resynchronization therapy (CRT). MATERIALS AND METHODS:122 patients (82.8% men) with NYHA functional class III-IV (mean age 54.8 ± 9.6 years) were enrolled. At baseline, 1, 3 months and each 6 months after implantation we evaluated clinical and echocardiographic parameters. In 28 patients the best decrease of left ventricular end-systolic volume (LVESV) was achieved up to 3 months (1.1 ± 0.9 months, I group-early response) and in 94 patients-after 3 months (22.6 ± 14.9 months, II group-late response). RESULTS:At baseline groups did not differ in main clinical characteristics, the proportion of atrial fibrillation, the presence of left bundle-branch block (LBBB), width of the QRS complex and parameters of mechanical dyssynchrony. Level of left ventricular ejection fraction (LVEF) and left ventricular volumes were comparable between groups. In the II group responders (decrease in LVESV ≥ 15%) were identified more frequently (90.4% vs. 60.7%; p=0.001), all patients with decrease of LVESV ≥ 30% (super responders) had late response.During follow-up period (33.2 ± 16.7 months) increase in LVEF and decrease in LVESV were more evident in patients with late response.In Kaplan-Meier analysis mortality in II group was significantly lower (3.2% vs 28.6%; p=0.001). Cox regression showed that LVESV (HR 1.012; 95% CI 1.004-1.021; P=0.005) and the time of response (HR 0.131; 95% CI 0.032-0.530; P=0.004) were associated with long-term mortality. CONCLUSION:Patients with early response to CRT show significantly lower improvement in LVEF and LVESV compared to patients with late CRT response. Super respone to CRT in associated with late functional improvement. Early response and greater LVESV are associated with higher mortality rate.