UNLABELLED: Apart from the classic role of vitamin D, its hormonal active form, calcitriol is also characterized by pleiotropic effects on various organs and tissues. For the last several years, many researchers have shown an association between deficiency of vitamin D and the risk of type 2 diabetes mellitus (T2DM). Recent investigations suggested the need of vitamin D supplementation in T2DM prevention. It was shown that vitamin D deficiency decreases insulin secretion. It was also observed that proper vitamin D supplementation may improve the ability of the cells of the islets of Langerhans to synthesize many proteins de novo and to convert proinsulin to insulin. Apart from regulating bone metabolism and also calcium and phosphate homeostasis, 1,25(OH)2D3 exerts antiproliferative and pro-differentiating effects on a wide variety of cell types. It also induces apoptosis of cancer cells and slows their proliferation. In a number of major studies the relationship between low vitamin D levels and increased risk of various cancers was observed. It concerns colorectal, lung, prostate, breast and ovarian cancer. It was observed that in patients with low serum vitamin D concentrations such disorders as ischemic heart disease, heart attack, stroke, cardiac arrhythmia, and hypertension were more frequent and mortality was significantly higher. These results led the researchers to consider vitamin D deficiency as a potential risk factor for cardiovascular diseases. The possible mechanism in the pathogenesis of cardiovascular diseases that may be related to low levels of vitamin D, is its adverse effect on the renin-angiotensin-aldosterone system (RAAS). Calcitriol is also an important determinant of muscle cell proliferation and differentiation, as well as inhibition of apoptosis. Vitamin D is synthesized in the skin. However, there are only a few food products that are rich in vitamin D3, e.g.: fish oils, fish and fortified-products, such as dairy products and margarines. Individuals who are vulnerable to vitamin D deficiency should be supplemented. KEY WORDS: vitamin D, diabetes mellitus, deficiency of vitamin D, cancer, cardiovascular diseases, muscle physiology.