Until recently, 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3))-the active form of vitamin D-was thought to function primarily as a regulator of calcium and phosphate metabolism. More diverse functionality was indicated by the discovery of the vitamin D receptor in tissues that are not involved in calcium and phosphate homeostasis. Detection of the vitamin D receptor in monocytes and activated T cells has sparked interest in the immunomodulatory properties of vitamin D. Here, we review the role of vitamin D in regulation of the immune system, and evidence for its involvement in the pathogenesis of inflammatory bowel disease.