Prevalence studies and investigations based on death certificates and disability assistance have shown that western Norway has been a medium-frequency area for MS. The prevalence of definite/probable MS on January 1, 1963 was 20.1/100,000. Based on the same diagnostic criteria, the prevalence of definite/probable MS had increased to 59.8/100,000 on January 1, 1983 in the county of Hordaland. We consider this increase of MS cases in the population to be due to real biologic changes, although factors affecting the figures were found. Most important was a reduction in the interval from onset to diagnosis. The rise in prevalence was also supported by an increase in the annual incidence of the disease. The incidence rate averaging 2 per 100,000 in the period 1953 to 1962 rose to 3.5 for 1968 to 1977. This marked rise in prevalence/incidence supports the concept that an exogenous factor is important in the pathogenesis of MS.