There is a growing number of diseases which prevalence can be associated with vitamin D deficiency. The link between low cholecalciferol concentration and bone diseases is well established, however there is also data suggesting that it may influence development and progression of different cancers and autoimmune diseases. The in vitro studies proved that the active vitamin D metabolite--1,25(OH)(2)D(3) may arrest the cell cycle progression, induce apoptosis as well as regulate T cells and antigen presenting cells function. Results of the in vivo experiments suggest that vitamin D deficiency accelerates development of autoimmune diseases and cancers in animals. Epidemiological studies imply that the vitamin D deficiency is also associated with the increased incidence of autoimmune diseases and cancers in people. The main determinant of vitamin D serum concentration in a human body is skin synthesis. The changes in the lifestyle, air pollution as well as a common use of sun screens caused that the contemporary European receives little sunlight compared to his ancestors. According to the recent epidemiological studies, the vitamin D concentrations in serum of people who live in high latitudes (above 34 degrees N/S), including Poland, is far from being sufficient. This paper reviews results of the recent studies concerning the potential role of the vitamin D in the development of cancers and autoimmune diseases, as well as provides guidelines for vitamin D supplementation.