In the epidemiological area of South Lower Saxony the prevalence of multiple sclerosis increased from 51/100,000 in 1969 to 99 in 1986. The mean annual incidence increased from 2.6 for the period from 1964-1974 to 4.6 for 1975-1985. This trend is significant (p = 0.0068). Diagnostic criteria, methods of case finding and analysis of the data remained stable. Minor necessary changes as for example due to new data protection laws and new handling of early cases by practising physicians can hardly explain this significant increase. In consideration of similar reports from all over the world biological exogenous factors are suspected but remain to be identified.