Summary: The basic role of vitamin D is related to calcium homeostasis and the regulation of bone metabolism. Recent years have shown its significance as an immunomodulatory agent. The incidence of vitamin D deficiency is increasing both in children and in adults, therefore it is important to maintain its proper serum concentration. Normal serum concentration and adequate vitamin D supplementation during pregnancy may help reduce the risk of asthma in children. In adults, it may reduce the risk of asthma and COPD exacerbation. Vitamin D supplementation is also important in the treatment and prophylaxis of osteoporosis related with the use of systemic glucocorticosteroids. At the same time, the use of vitamin D without medical supervision and monitoring may lead to adverse and toxic effects. In the group of patients with sarcoidosis, uncontrolled vitamin D supplementation is associated with the development of hypercalcemia.