Background. Patients with heart failure who have a reduced left ventricular ejection fraction in combination with delayed intraventricular conduction need cardio-resynchronization therapy (CRT). There is still no generally accepted explanation of the effect of cardio-resynchronization therapy on myocardial contractility. We hypothesized that increasing the width of the QRS complex as a result of the left bundle branch block leads to an increase in the duration of energy-consuming systole of the left ventricle and shortening of its diastole, which is one of the mechanisms for the development of heart failure. We assume that a decrease in the width of QRS with cardio-resynchronization therapy reduces the time of mechanical work of the heart and lengthens the “rest” period. Consequently, the narrower is the QRS complex obtained with cardio-resynchronization therapy, the greater is the positive effect of the cardio-resynchronization therapy. Objective. To identify the relationship between the width of the QRS-stimulated complex and the positive effect of cardio-resynchronization therapy. Material and methods. The single-centre retrospective study included 46 patients with left bundle branch block, dilated cardiomyopathy and sinus rhythm. The mean follow-up was 989±807 days. Patients' data were analyzed, the width of the QRS complex was evaluated before and after the operation, and the ultrasound data were assessed. The patients were divided into two groups: non-responders (an increase in ejection fraction less than 10%, a decrease in end-diastolic volume less than 15%, or negative dynamics), and responders (an increase in ejection fraction by 10% or more, a decrease in end-diastolic volume by 15% or more). Results. 31 persons were assigned to the group of responders, the group of non-responders included 15 patients. Initially, the groups did not differ by age, sex, body mass index, comorbidity, end-diastolic volume, QRS complex width (168.1±15.7 versus 172.3±20.8 msec, p=0.442). Overall mortality from all causes during the observation period was 17.4% (8 patients). In the group of non-responders, mortality was significantly higher (6 patients, 37.5%) than in the group of responders (2 patients, 6.5%). Patient survival, depending on the width of the QRS complex after implantation of devices for cardio-resynchronization therapy, tended to differ in patients with QRS≤130 msec and others, but the reliability criteria were not achieved (p=0.254). In the non-responders group, the QRS width after implantation was significantly greater than that of the responders (146.3±16.9 versus 137.0±13.4 msec, p=0.049). Conclusion. In our series of observations, a smaller width of the stimulated QRS complex was associated with a positive effect of cardio-resynchronization therapy.