Multiple sclerosis (MS) is an inflammatory and neurodegenerative disease of the central nervous system. Genetic and environmental risk factors are likely operative in MS pathobiology. MS prevalence varies from very low rates in peri-equatorial regions, to over 100/100 000 population in temperate regions distant from the equator- prompting consideration of limited sunlight and vitamin D insufficiency as potential risk factors for MS. In a large study of MS patients living in temperate countries, significantly fewer were born in November and significantly more in May, suggesting that seasonally-related environmental risk factors may already even be operative pre- or perinatally. Several retrospective case-control studies have shown that greater sun exposure during childhood and adolescence was associated with a reduced risk of adult-onset MS. Individuals reporting oral ingestion of vitamin D supplements, or ingestion of a diet rich in vitamin D-containing foods also have a lower MS risk. The increasing recognition of MS in children and adolescents has prompted study of vitamin D status in children with confirmed MS, and in children who have experienced a first demyelinating event (50% of whom will ultimately be diagnosed with MS). In our national pediatric demyelination study in Canada, low serum 25-hydroxyvitamin D (25(OH)D) concentrations at the time of a first demyelinating attack predict MS risk- with a 20% reduction in risk for every 10 nmol/l increase in 25 (OH)D concentration. These data, the available literature, and potential biological and therapeutic considerations will be presented.