Evidence has emerged in the past few years that provides support for a protective action of vitamin D against the risk and progression of multiple sclerosis (MS). The effect of exposure to ultraviolet radiation and other natural sources of vitamin D on MS prevalence and disability is the focus of two recent population-based studies. In Norway, Kampman et al. conducted a retrospective analysis of 152 patients with MS and 402 controls matched for age, sex and place of birth, and found that increased time spent outdoors in summer was significantly associated with a decreased risk of develop ing MS later in life. The association was strongest for those aged 16–20 years (odds ratio 0.55; P = 0.001), confirming the results of migration studies that have identified this as the period of life during which risk of developing MS is established. In the group with the lowest levels of exposure to summer sunlight, there seemed to be a protective effect of high consumption of fish or taking fish oil; the authors suggest that the existence of a high-fish diet in a location with low sunlight might explain why the latitude gradient of MS risk is not seen in Norway. Although the study design precluded the identification of a causal relationship, the results indicate that the risk of MS is influenced by climate and diet, both of which affect vitamin D levels in the body. In a second study, van der Mei et al. examined the prevalences of vitamin D insufficiency and deficiency in a Tasmanian population of 136 patients with MS and 272 controls matched for age and sex. In addition, they assessed possible determinants of vitamin D status in patients with MS. The researchers found that vitamin D deficiency and insufficiency were highly preva lent in patients with MS (7.4% and 43.3%, respectively), as well as in controls (6.1% and 40.7%). Notably, in comparison with controls, the high-disability patients with MS were more likely to have lower serum levels of 25OH-vitamin D (odds ratio 3.07, 95% CI 1.37–6.90), but the lower-disability patients were not (odds ratio 0.87, 95% CI 0.41–1.86). An increasing level of disability in patients with MS was strongly correlated with lower levels of serum vitamin D (r = –0.44; P <0.0001) and with lower levels of recent sun exposure (r = –0.39; P <0.0001). Nearly half of the risk of vitamin D deficiency in the high-disability group was attributable to lower recent sun exposure, perhaps because of sun-avoidance behavior in this group. There was a positive, although not significant, association between fish intake and serum 25OH-vitamin D levels only in the group with particularly low sunshine exposure, reflecting the fact that most vitamin D is produced via radiation and not ingested. Prospective, longitudinal studies are needed to determine whether vitamin D deficiency has a direct effect on the level of disability of patients with MS, but van der Mei et al. suggest that levels of vitamin D should be monitored and corrected as part of the clinical management of MS.