An epidemiological field-survey on multiple sclerosis (MS) was carried out in the southern part of the state of Hesse (West Germany). Out of 472 patients primarily ascertained, 75% could finally be accepted as definite or probable MS cases. Cross-checking of the 3 sources for case-finding (practitioners and specialists, neurological hospital, MS society) revealed that case-finding in all and especially that of definite and probable MS cases was rather complete. The overall prevalence rate was 58.3, the annual incidence 2.15 and the mortality rate 2.0 per 100,000. Male patients showed a faster course than females. The prevalence in immigrants who mostly originated from Mediterranean countries, was significantly lower (14.3 per 100,000) than the prevalence in the German population (63.1 per 100,000).