The degree of immune dysfunction precipitated by an inciting infection is thought to correlate with the severity of sepsis. Recently, key cells of the immune system were shown to express the vitamin D receptor (VDR) [9]. Additionally, macrophages and neutrophils activated through VDR up-regulate expression of endogenous antimicrobial peptides (AMPs) that are active against a broad spectrum of infectious agents [10]. AMPs, influenced by vitamin D status, likely represent an important first-line of defense against microbial invasion. While circulating 25-hydroxyvitamin D (25OHD) is the most abundant vitamin D metabolite [11], under normal circumstances, 85-90% of 25OHD is tightly bound to vitamin D binding protein (DBP) and is unavailable to activate innate immune responses during acute stress [12]. The remaining 10-15% is more readily bioavailable (b25OHD), and is composed of the free and albumin-bound components of 25OHD. Fluid loading, albumin wasting, and increased DBP expression during critical illness may affect 25OHD homeostasis [13].