Both calcium and vitamin D play important roles in cardiac muscle contraction and performance. In this cross-sectional study, we evaluated the status of serum calcium, PTH and 25(OH)D3 and their correlation with left ventricular Function and NYHA Functional class in 95 heart failure patients referred to Shahid Chamran Hospital, Isfahan, Iran, by colorimetric, immunoradiometric, and Immunochemiluminescent assays, echocardiography and interview respectively. The study was performed between Oct 2007 and Feb 2008. Twenty eight women and 67 men of functional classes 1, 2, or 3 participated in the study. Mean (SD) of age of the participants was 62(11) years. Mean (SD) serum calcium and 25(OH)D3 were 2.41(0.16) mmol/L and 56.78(51.33) nmol/L, respectively. The overall prevalence of low vitamin D status was 84.2%. There was no correlation between serum calcium and 25(OH)D3 with LVEF. Interestingly, patients with hyperparathyroidism (serum PTH>65 ng/L) had lower LVEF (27% versus 32.5% p = 0.03). NYHA functional class was worse in patients with hyperparathyroidism (p = 0.08). Hypovitaminosis D is very prevalent in heart failure patients. Hyperparathyroidism in these patients may adversely affect cardiac function. Vitamin D3 might serve as an adjunctive treatment for heart failure patients.