MULTIPLE SCLEROSIS (MS) is one of the most common primary diseases of the nervous system and one whose cause remains totally unknown. Detailed study of its anatomic and chemical pathology has not yet provided answers necessary for a basic understanding of the illness. With presently available methods, one major aspect of the disorder that has not been thoroughly evaluated is its epidemiology. Epidemiology of a disease is the study of the distribution and course in relation to environmental and genetic factors with the goal of clarifying its cause, that is, the who, what, when, and where, in order to find out why. Epidemiology has only recently broken out of the rather narrow confines of acute infectious disease into the problems of chronic illness. The questions remain the same although the methods differ. In considering the distribution of a disease, the basic axiom is that the risk of a given illness