Receptors for vitamin D are present almost in all body tissues. Vitamin D does not only act as a regulator of calcium and phosphorus metabolism, but also a substance which modulates the immunity system function, regulates cell proliferation. Genetic, epidemiological and experimental data indicate a possible role of vitamin D deficiency in the development of some diseases, mainly of autoimmune diseases and tumours. The benefit of curative intake of vitamin D has so far not been proven. There is no reason why the deficiency of vitamin D should not be covered. The substitution should start with the replenishment of the lacking amount of vitamin D (which may even be several hundred thousand units) and involve ongoing intake of the recommended substitution dose of 800-1000 vitamin D units per day.