Neuroepidemiology has been important in providing clues about the cause and pathogenesis of multiple sclerosis. In this review, we update the incidence and prevalence rates of multiple sclerosis in Olmsted County, Minnesota, and examine the potential role of viruses, exposure to animals, toxins, trauma, and diet in the development of this disease. Diseases of probable autoimmune nature have also been linked to multiple sclerosis. These descriptive data may contribute to the formulation of testable specific hypotheses about the pathogenesis and treatment of multiple sclerosis and other demyelinating diseases.