Background: Cardiac resynchronization therapy (CRT) improves mortality and induces reverse remodelling in chronic heart failure (HF) patients with reduced ejection fraction and ventricular dyssynchrony. However some patients do not improve despite of optimal medical therapy and optimal indications for CRT. Therefore finding biomarkers suitable for identification of those patients is crucial. Vitamin D plays a classical hormonal role in the regulation of bone metabolism. Vitamin D and its metabolites also have physiological functions in wide range of nonskeletal tissues. Numerous studies have demonstrated novel actions in pathways that are integral to cardiovascular function and disease as well, such as inflammation, thrombosis and activation of renin-angiotensin system. Based on recent studies low levels of vitamin D seem to directly contribute to pathogenesis and worsening of HF. We aimed to assess the vitamin D level on clinical outcomes of HF patients undergoing CRT. Methods: We prospectively enrolled 136 HF patients undergoing CRT. Total plasma vitamin D levels were measured at baseline and 6 months later, using Elecsys Vitamin D total assay on Cobas Integra 400 (Roche Diagnostics). Primary end-point was 5-year all-cause mortality, secondary end-point was clinical responsiveness, defined by 15% decrease of left ventricular end systolic volume (LVESV) after six months. Results: After five years 58 patients reached the primary end-point, while 66 were considered as non-responders. Vitamin D levels remained unchanged after six months of CRT. Vitamin D levels under 24.23 ng/ml were associated with lack of reverse remodelling (p = 0.045) although the prediction was not independent. On the other hand vitamin D deficiency proved to be an independent predictor of 5-years mortality after adjusting to all significant baseline predictors including age, beta-blocker therapy, left brundle branch block and baseline NT-proBNP levels. Conclusions: Our study confirmed that vitamin D deficiency has a significant impact in heart failure patients. We found, that it is an independent predictor of long term mortality in patients undergoing CRT and is associated with lack of response as well. Therefore monitoring vitamin D status of heart failure patients could be of clinical significance.