Past sun exposure and vitamin D supplementation have been associated with a reduction in the risk of MS. We measured the serum concentration of 25-hydroxyvitamin D (25[OH]D) at the time of MS diagnosis in 40 MS patients and 40 controls. We found no difference in the serum levels of 25(OH)D between MS patients and controls when all samples or samples obtained during winter months were compared, but MS patients had significantly lower serum 25(OH)D concentrations in June to September than the controls. The vitamin D stores were adequate for bone metabolism (> 37 nmol/L) in 70% of MS patients throughout the year and within the hypovitaminosis level (< 37 nmol/L) in 30% of MS patients at some time of the year. During MS-relapses, 25(OH)D levels were lower than in remission, but mostly within the reference range observed in relation with normal bone metabolism. We conclude that the vitamin D stores in most MS patients are adequate for their normal bone metabolism. However, lower vitamin D levels during MS relapses than in remission suggest that vitamin D could be involved in the regulation of the clinical disease activity of MS. The optimal serum levels of vitamin D for the regulation of immune responses remain to be determined.