The epidemiology of multiple sclerosis (MS) is characterized by the fact that there is an uneven distribution of the disease throughout the world. The two most commonly used indices of its frequency are the incidence and prevalence rates. The incidence rate reflects, to a great extent, the influence of environmental factors in triggering the clinical manifestations of the disease, if it is based upon the actual date of the initiation of symptoms, rather than the date of diagnosis. The prevalence rate is currently based on the date of diagnosis and includes all MS patients who are alive on a particular date, without regard to their ethnic origin, the site and the duration of residence or any other factors that may have influenced the acquisition of the disease. We propose that in order to make the concept of the prevalence rate possibly more meaningful, the term should refer, retrospectively, to all patients whose symptoms eventually led to the diagnosis of MS, even though the diagnosis was not yet established on an earlier prevalence day. In addition, only patients of the same ethnic background who have spent their prepuberal years in the geographical area under study should be included. We are calling this measure the onset-adjusted prevalence rate. Another problem encountered in epidemiological studies of MS relates to the common practice of adjusting prevalence data obtained for age and sex in one area to what is referred to as a "standard" American (or world) population, groups of great ethnic and age diversity. It is also curious that in many studies the bases for comparison of populations are census data obtained many years previously. We suggest that age and sex adjustment should be applied only to similar ethnic groups born and raised under different environmental conditions. We believe that data obtained by calculating an onset-adjusted prevalence rate restricted to a homogeneous group of patients sharing the same environment during the prepuberal years may provide valuable etiological clues.