Epidemiological and experimental evidence suggest that a poor vitamin D status is associated with an increased risk of developing multiple sclerosis (MS), and also an unfavorable disease course. Vitamin D may exert relevant effects both on the immune system and on resident cells within the CNS. The data from clinical trials is, however, restricted, and does not allow any conclusion on the effect of high-dose vitamin D supplementation on disease course. The results from sufficiently powered studies will not be available for at least 2 years. MS patients are, however, prone to develop osteoporosis and have increased risk of fractures. Therefore, the authors advise that the serum concentration of 25-hydroxyvitamin D is monitored in order to prevent bone deficit, and that a serum level of 75-125 nmol/l is targeted. This level is sufficient for maintenance of bone health, is not known to be associated with adverse events, and is in the range that has been associated with low risk of developing MS and low disease activity.