The aim of this project was to assess the clinical state, tolerance for physical exertion, NT-proBNP levels and quality of life (QoL) of patients after initiation of cardiac resynchronization therapy (CRT) as well as to verify the working hypothesis that QoL correlates with improvement in clinical, electrocardiographic, echocardiographic and biochemical indicators. The aim of this project was to approach this problem by attempting to answer the question of whether patients who respond to CRT obtain an improvement in QoL in comparison with patients who do not respond to CRT. The study was carried out on 60 patients with chronic heart failure (NYHA III a IV) despite of optimal pharmacotherapy and with electrocardiogram QRS duration greater than 120 ms, LVEF ≤ 35% and LVEDd ≥ 55 mm. A subjective and objective examination, laboratory tests including measurement of NT-proBNP levels, a resting electrocardiogram, six-minute walk test (6-MWT) and echocardiogram were carried out on the members of the study group before implantation of the CRT device. All of the patients also completed the PGWB quality of life assessment test. Three months after implantation of the CRT device all of the patients made a follow-up clinical visit during which the tests were repeated. 57 patients completed the three month observation period. Three of the patients from the initial group died. Statistically si ; gnificant decreases in ailments related to heart failure, increases in physical exertion tolerance, decreases in the width of the QRS complexes were observed. A decrease of NT-proBNP levels was also observed three months after the intervention; however statistically this result was not significant. A significant improvement in QoL, LVEF and reduction of LVEDV, LVESV were also noticed. Using improvement/no change QoL criteria, it was seen that the studied groups initially differed in QoL. The group of patients who experienced an improvement in QoL three months after implantation of the CRT device initially exhibited a lower QoL. Depending on the criteria one uses, the percentage of patients responding to CRT differed significantly within this same group of patients. If we accept that the criteria for responding to CRT is increase in 6-MWT≥10%, than we can distinguish a group of patients who besides exhibiting a positive response to heart resynchronization therapy also obtained an improvement in quality of life.