Vitamin D3, combined with its nuclear receptor, regulates more than 900 genes, which is the reason why its effect is pleiotropic. Among other effects, it influences the immunological system. Its deficit may be one of the environmental factors taking part in the development of auto-immunological diseases. The studies reveal that, among others things, inflammatory bowel diseases occur in higher latitudes, with lower exposure to solar radiation and with decreased production of vitamin D3 in the skin. Patients with inflammatory bowel disease have vitamin D3 deficiency more frequently. The application of vitamin D3, especially among adults with inflammatory bowel diseases, positively influences bone turnover markers and mostly due to its influence on immunological processes, vitamin D3 may be useful in the treatment of the primary health condition. It is necessary to determine the dosage range, as well as the optimal level of vitamin D3 metabolite-25OHD3, where the immunosuppressant effect is the best, with no toxic effects. The basic function of vitamin D3 in the body is its influence on keeping the homeostasis of calcium metabolism, preventing the appearance of rickets, osteomalacia and osteoporosis [1,2]. Vitamin D comes from food -ergocalciferol (vitamin D2), and mostly it is synthesized in the skin under the influence of ultraviolet radiation B (UVB) -cholecalciferol (vitamin D3). Next, in the liver, it is hydroxylated into its inactive form 25 OH D3 (calcidiol) and in the kidney into its active form -1,25 (OH)2 D3--calcitriol. Except for the cells of cuticle of proximal tubule in the kidney, the activity of 1-alfa-hydroxylase have also, among others, antigen presenting cells (APCs), parasympathetic ganglions, hair follicles, cerebral cortex cells, pancreatic islets [3]. Vitamin D acts through its receptors (vitamin D receptors -VDr), which may be found in the majority of organs: in the alimentary tract, bones, the brain, the chest, muscles and others. [3]. Vitamin D combined with the receptor is the regulator of more than 900 genes. In this way it regulates, among others, the calcium metabolism, intestinal calcium transport, as well as cell proliferation and differentiation [4]. An analysis of microarrays and the genome screening of vitamin D3 receptors (VDrs), which are nucleus receptors and direct transcription factors, detected the molecular base of physiological effects of vitamin D3-both its influence on calcium metabolism, immunological system as well as its antitumor activity [4].