Multiple sclerosis (MS) is an autoimmune disease of unknown etiology whose treatment is of limited efficiency and therefore has a high social burden. As it has been suggested that myelin destruction model, the clinical manifestation and the potential of therapeutic response in MS are correlated, it is quite justifiable that we study various factors (genetic, hormonal, environmental) that take part in the autoimmune process in order to improve the control over the disrupted immune regulation. Results from epidemiological and clinical studies clearly suggest that changes in vitamin D serum concentrations are correlated with the magnitude of the risk of developing MS, the phases of relapse and remittance and with gender differences in vitamin D metabolism. Experimental and clinical studies also have established that 25-hydroxy vitamin D (25(OH)D) and 1,25-dihydroxy vitamin D (1,25(OH)2D) exert an immunomodulatory effect in the central nervous system and peripheral organs of the immune system. The standard reference range of vitamin D concentration in serum is 50-80 nmol/l--it provides normal calcium metabolism. Issues that are discussed include the vitamin D serum concentration needed to suppress the aberrant immune response in MS patients; a subgroup of MS patients suitable for vitamin D treatment, the vitamin D being applied in optimally effective and safe dosage. MS prevalence rate in Bulgaria has increased two-fold in 17 years but this is a rather short interval to be able to assume that the gene pool of the population changes. Thus further studies on possible interactions between different environmental factors and these factors' role in the disease pathogenesis are justified and necessary.