Evidence of the role of vitamin D in the regulation of T and B cells, macrophages, dendritic cells, and keratinocytes continues to accumulate and provides a link between vitamin D and many autoimmune diseases, including Crohn's disease, juvenile diabetes mellitus, multiple sclerosis, asthma, and rheumatoid arthritis. Considering the influence of vitamin D on the immune system, it may have potential as a treatment for immune-mediated diseases, even if additional research is required to better quantify dosage. But the biggest obstacle to its clinical use is its potent hypercalcemic effect. The calcium status of the host may influence the effect of vitamin D on immunity.