AIM 1: HD patients are profoundly vitamin D deficient, yet ~60% of incident hemodialysis patients in the US are not treated with active vitamin D. Vitamin D is linked with cardiovascular health, and although higher serum vitamin D levels are associated with improved survival in different settings (e.g., cancer, infection), no large study has examined whether baseline vitamin D levels are associated with HD mortality. Measuring serum vitamin D levels among HD patients is sporadic since the utility of this measurement as it relates to dialysis mortality is unclear.