BACKGROUND Although multiple sclerosis (EM) has the characteristics of a disorder which is complex to study epidemiologically, many papers have been published on its genetic epidemiology, and these have given a great deal of information regarding the aetiological factors of the disorder. METHODS These epidemiological investigations have also studied the incidence and prevalence of EM in each zone, the different geographical distribution, its variation with immigration, relation to race and sex, the existence of possible epidemics or groups and its family aggregation. They have supported the environmental aetiological factor of EM, the geographical gradient of the frequency, possible epidemics or groups, changes with migrations and concordance in identical twins of less than 100%. However, arguments in favour of a genetic aetiological factor have been supported by racial differences in the frequency of this disorder, the existence of resistant ethnic groups in spite of the migrations and the increased concordance in monozygotic twins. CONCLUSIONS The results of studies on the genetic epidemiology of EM support the multifactorial nature of its aetiology, with a polygenic type of genetic susceptibility, although this alone does not justify the development of EM. Some environmental factor, as yet unknown, is necessary for the disease to develop in a genetically susceptible person. Molecular genetic studies are adding further knowledge about this genetic susceptibility, although the genes involved have not yet been conclusively identified because of the complexity of EM.