Severe sepsis is a significant public health problem with limited treatment options. A 2010 NIH workshop on sepsis identified deep gaps in understanding innate host immune responses in sepsis and emphasized the need for novel therapies [18]. The proposed trial is well powered to investigate whether immunomodulation with vitD3 improves survival after severe sepsis. By optimizing endogenous host responses to infection, the investigators anticipate that vitD3 supplementation will augment the life-saving impact of current therapies for severe sepsis. Moreover, the investigators will investigate several vitamin D-related biomarkers, which may identify future therapeutic targets. The trial will be performed by a highly-experienced team that includes experts in vitamin D, sepsis, critical care, and clinical trials; they have successfully carried out several clinical studies of vitamin D in critically ill patients.