The identification of vitamin D receptor in cells involved in the immune response and the discovery that activated dendritic cells produce vitamin D hormone suggested that vitamin D could exert immunoregulatory effects. Patients with autoimmune diseases such as multiple sclerosis, rheumatoid arthritis and systemic lupus erythematosus (SLE) show low 25-OH vitamin D serum levels. In particular, SLE patients have multiple risk factors for vitamin D deficiency and disease severity seems correlated with lower 25-OH vitamin D serum levels. Treatment of vitamin D deficiency could be particularly important in SLE patients due to concomitant insults on their tissues such as bone, and in view of the possible immunomodulatory effects exerted by vitamin D.