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The inverse relationship of multiple sclerosis (MS) to sunlight exposure was proposed at least 50 years ago but conclusive evidence is unavailable.  Since the proposal was made, many researchers have jumped to the conclusion vitamin D mediates this effect. Vitamin D is a pro-hormone produced in the skin by UV irradiation and/or sun exposure.   In addition, an inverse relationship between plasma levels of the vitamin D metabolite, 25(OH)D₃, and a reduced incidence of MS has been noted.  As a result of the relationship clinical trials have been carried out to determine if vitamin D supplementation can suppress the disease. The results have not supported this idea. In an animal model of MS (experimental autoimmune encephalomyelitis or EAE) the vitamin D hypothesis was refuted.  UV irradiation, on the other hand, consistently reduces the incidence and severity of EAE.  Because EAE is a widely accepted model of human MS, clinical trials testing the efficacy of the narrow band UV light in suppression of MS appear warranted. If in fact this narrow band light does suppress human MS, isolation and identification of the compound produced by this light should be initiated.