The aim of the present study was to investigate whether vitamin D supplementation could improve biochemical findings and functional capacity of patients with heart failure (HF). One hundred patients with New York Heart Association (NYHA) class I through III HF were included in this prospective study and their 25-hydroxyvitamin D levels were evaluated. Only 6% of the participants had a sufficient serum concentration of 25(OH) D >30 nmol/L. Patients with insufficient or deficient serum levels of 25(OH) D (<30 ng/mL and <20 ng/mL, respectively) received oral vitamin D3 (cholecalciferol) for a total period of 4 months. Vitamin D supplementation increased mean serum concentration of 25(OH) D from 12.63±7.60 nmol/L to 54.49±18.01 nmol/L (P<.001). After vitamin D supplementation, the serum level of pro-brain natriuretic peptide markedly decreased (P<.001). Cholecalciferol significantly decreased high-sensitivity C-reactive protein level (P<.001). Restoration of serum 25(OH) D level was also associated with substantial improvement in NYHA class (P<.001) and 6-minute walk distance (P<.001).