Introduction: The Danish Multiple Sclerosis (MS) Registry was established in 1956. Content: The register comprises data on all Danes who had MS in 1949 or who have been diagnosed since. Data on new cases and updated information on persons with an MS diagnosis already notified are continuously collected and classified according to standardized diagnostic criteria. Validity and coverage: The completeness and validity is high (91% and 94%, respectively). CONCLUSION: In January 2005 the register included data on 20,652 persons with a valid MS diagnosis, of whom 9,377 were alive. The register is the only existing nationwide MS register and covers a period of more than 50 years.