Mortality rates from multiple sclerosis show a well-known north-south gradient, both within the United States and internationally. Mortality rates from prostate cancer show a similar gradient and are significantly correlated with multiple sclerosis (MS) mortality and MS prevalence. This finding adds prostate cancer to the set of diseases whose geographic distributions are significantly correlated with MS and whose members include colon cancer, dental caries, and Parkinson's disease. Review of the literature indicates that these clinically dissimilar diseases may share an aberration in vitamin (hormone) D. Recent evidence demonstrating a multi-faceted role for vitamin D in immunoregulation suggests that a vitamin D aberration may also contribute to the etiology of MS. A vitamin D hypothesis can illuminate several unexplained features of the epidemiology of MS and suggests opportunities for epidemiologic, laboratory, and clinical investigation.