The renin-angiotensin system (RAS) is a regulatory system that plays an essential role in patients with chronic heart failure (CHF). Plasma renin activity (PRA) is a strong and independent predictor of outcome, also in the presence of ACE inhibitors (ACE-i) and/or angiotensin receptor blockers (ARBs). Recently, it has been shown that vitamin D regulates renin transcription by activating the vitamin D receptor (VDR). Thus, specific activation of the VDR represents a novel target for therapeutic intervention in CHF. Currently, clinical data are lacking. The investigators aim to investigate the effect of the administration of vitamin D in patients with CHF.