Recent evidence supports an association between vitamin D deficiency and hypertension, peripheral vascular disease, diabetes mellitus, metabolic syndrome, coronary artery disease, and heart failure. The effect of vitamin D supplementation, however, has not been well studied. We examined the associations between vitamin D deficiency, vitamin D supplementation, and patient outcomes in a large cohort. Serum vitamin D measurements for 5 years and 8 months from a large academic institution were matched to patient demographic, physiologic, and disease variables. The vitamin D levels were analyzed as a continuous variable and as normal (≥30 ng/ml) or deficient (<30 ng/ml). Descriptive statistics, univariate analysis, multivariate analysis, survival analysis, and Cox proportional hazard modeling were performed. Of 10,899 patients, the mean age was 58 ± 15 years, 71% were women (n = 7,758), and the average body mass index was 30 ± 8 kg/m². The mean serum vitamin D level was 24.1 ± 13.6 ng/ml. Of the 10,899 patients, 3,294 (29.7%) were in the normal vitamin D range and 7,665 (70.3%) were deficient. Vitamin D deficiency was associated with several cardiovascular-related diseases, including hypertension, coronary artery disease, cardiomyopathy, and diabetes (all p <0.05). Vitamin D deficiency was a strong independent predictor of all-cause death (odds ratios 2.64, 95% confidence interval 1.901 to 3.662, p <0.0001) after adjusting for multiple clinical variables. Vitamin D supplementation conferred substantial survival benefit (odds ratio for death 0.39, 95% confidence interval 0.277 to 0.534, p <0.0001). In conclusion, vitamin D deficiency was associated with a significant risk of cardiovascular disease and reduced survival. Vitamin D supplementation was significantly associated with better survival, specifically in patients with documented deficiency.