Abstract Background and aim Low plasma vitamin D levels have been associated with heart failure (HF). This research attempts to explain the role of vitamin D supplementation on myocardial function in elderly patients with HF. Methods and results Twenty-three chronic HF patients were randomized in a small parallel group, double-blind, placebo-controlled trial. All patients, with a mean age of 74 years and vitamin D levels <30 ng/mL, received 800,000 IU (4000 IU/daily) of cholecalciferol or placebo for 6 months. The outcomes measured at baseline and after 6 months were ejection fraction (EF) and other echocardiography parameters, carboxyterminal propeptide of procollagen type I (PIP), natriuretic peptides, lipid profile, renin, parathyroid hormone, blood pressure, and body mass index (BMI). In 13 patients under active treatment for 6 months, mean plasma 25-hydroxy vitamin D concentrations (15.51 vs. −1.40 ng/mL, p  < 0.001) and plasma calcium (from 9.3 to 9.6 mmol/L, p  < 0.05) increased significantly. However, other biomarkers of bone metabolism did not differ between the treatment and placebo groups. EF increased significantly in the intervention group (6.71 vs. −4.3%; p  < 0.001), and the serum concentration of PIP increased only in the placebo group after 6 months (1140.98 vs. −145 mcg/L; p  < 0.05). Systolic blood pressure was lower after 6 months of cholecalciferol treatment (from 129.6 to 122.7 mm Hg, p  < 0.05). No significant variations were observed for other parameters. Conclusions Six months of vitamin D supplementation significantly improves EF in elderly patients with HF and vitamin D deficiency.