Multiple sclerosis is an inflammatory disease of the central nervous system of unknown aetiology. Its prevalence varies by ethnicity and place: persons of northern European descent are at increased risk while persons living at lower latitudes appear to be protected against the disease. The Danish Multiple Sclerosis Registry is a national registry established in 1956 after a population-based survey which receives information from numerous sources. It is considered to be more than 90% complete, with a validity of 94%. Using data from the Registry, we calculated prevalences per 100,000 inhabitants. The standardized prevalence of multiple sclerosis increased from 58.8 (95% confidence interval: 54.9-62.7) in 1950 to 154.5 per 100,000 (95% confidence interval: 148.8-160.2) in 2005, and the female to male ratio increased from 1.31 in 1950 to 2.02 in 2005. The increase in prevalence is due to both increased survival of multiple sclerosis patients and an increased incidence rate. The rise in prevalence in the past 50 years is probably due more to environmental factors than to genetic changes in the Danish population. Among women, environmental changes could include older age at first birth, use of oral contraceptives, or changes in sun behaviour and/or vitamin D status.