Type 1 diabetes mellitus (T1DM) is a chronic autoimmune disease that develops as a result of a genetic predisposition and environmental factors. Literature data indicate that the suboptimal status of vitamin D can be considered as a risk factor for the development of T1DM, especially at some stages of life. Adequate vitamin D supplementation in childhood may provide a protective effect and reduce the risk of developing T1DM at a later age. Pathogenesis of T1DM predisposes to abnormalities in the metabolism of vitamin D, including the development of vitamin D deficiency. Moreover, the immunomodulating effect of calcitriol (induction of immune tolerance and T-cell anergy, impaired B-cell activity and antibodies production) suggests the therapeutic potential of vitamin D in autoimmune diseases, including T1DM. A number of studies have demonstrated the positive clinical effects of various vitamin D preparations with respect to maintaining residual -cell function, improving glycemia control in patients with T1DM. Determining the optimal doses of vitamin D for patients with T1DM may contribute to disease control and prevention of complications.