['Vitamin D is a fat-soluble hormone necessary for calcium homeostasis. Recently, studies have demonstrated that vitamin D may be important to the health of the cardiovascular system.', 'Adults ≥ 50 years of age with heart failure were recruited for assessment of serum 25-hydroxyvitamin D (25OHD) concentrations. Cardiopulmonary exercise testing was used to assess functional capacity. Proximal muscle strength was evaluated with a Biodex isokinetic dynamometer [corrected] (Biodex, Shirley, NY), and health status was assessed with the Kansas City Cardiomyopathy Questionnaire. Univariate associations between physical performance and health status measures and 25OHD followed by a linear regression model were used to study associations, adjusting for other potential explanatory variables.', 'Forty adults 67.8 ± 10.9 years of age (55% women and 57.5% African American) with mean ejection fraction 40% were analyzed (New York Heart Association class II in 70% and class III in 30%). Comorbidities included 77.5% hypertension and 47.5% diabetes. The mean 25OHD concentration was 18.5 ± 9.1 ng/mL, and mean peak Vo₂, 14 ± 4 mL/kg/min. In univariate regression analysis, 25OHD was positively associated with peak Vo₂ (P = .045). Multivariable regression analysis sustained positive association between 25OHD and peak Vo₂ (P = .044) after adjusting for age, race, and respiratory exchange ratio (adjusted R² = 0.32). Association between proximal muscle strength with the 25OHD concentration was not significant. The Kansas City Cardiomyopathy Questionnaire physical limitation domain score was negatively associated with 25OHD (P = .04) but was not sustained in multivariable analysis.'] Is serum 25-hydroxyvitamin D concentration associated with functional capacity in older adults with heart failure?