The Danish Multiple Sclerosis Registry was established in 1948 in continuation with a nationwide survey of the prevalence of Multiple Sclerosis (MS) in Denmark. The register has since collected information on MS patients from all Danish departments of neurology, practising neurologists, MS rehabilitation centres, the National Patient Registry, the Danish MS Society, and departments of neuropathology. The registry is linked with the Danish Central Population Registry. The completeness has been estimated at more than 90%. All cases are reclassified by two neurologists as to diagnosis and year of onset. 12 070 cases with a confirmed diagnosis of MS are kept in the databases. They were prevalent in 1949 or have had onset in the period 1948 - 1993. The registry is continuously updated with new information on registered cases and new cases. The crude average annual incidence rate 1980 - 89 was 4.99/105; the prevalence rate was 112/105 by 1 January 1990. Cross-linking with other registers have enabled analytical prospective epidemiological studies, and the registry has provided population based unbiased samples of patients for a number of clinical studies.