OBJECTIVE: To assess longitudinal follow-up of the incidence of multiple sclerosis (MS) through five decades and estimate the prevalence rate in Hordaland County, Norway, on January 1, 2003. METHODS: All patients with MS diagnosed from 1953 to 2003 were identified in the patient records of the Department of Neurology, Haukeland University Hospital, Bergen, Norway. The diagnostic criteria of Poser et al. were applied and only patients with definite and probable MS were included. The study comprises 912 patients, and 666 patients with MS were living in Hordaland on January 1, 2003. The annual incidence rates for the years 1953 to 2003 were calculated. RESULTS: The total crude prevalence rate on January 1, 2003, was 150.8 per 100,000 population: 191.3 per 100,000 among women and 109.8 per 100,000 among men. The annual incidence of MS increased from 1.8 per 100,000 in 1953 to 1957 to 6.0 per 100,000 in 1993 to 1997. CONCLUSIONS: Hordaland County, Norway, has changed from a low-risk to a high-risk area for multiple sclerosis (MS) during the last 50 years. During the last 25 years, the incidence of MS has been stable rather than increasing. Systematic longitudinal follow-up studies are essential to calculate reliable prevalence and incidence rates in MS. The results suggest that both methodologic and environmental factors are essential in determining the distribution of MS. OBJECTIVE: To assess longitudinal follow-up of the incidence of multiple sclerosis (MS) through five decades and estimate the prevalence rate in Hordaland County, Norway, on January 1, 2003. METHODS: All patients with MS diagnosed from 1953 to 2003 in the patient records of the Department of Neurology, Haukeland University Hospital, Bergen, Norway. The diagnostic criteria of Poser et al. were applied and only patients with definite and probable MS were included. The study includes 912 patients, and 666 patients with MS were living in Hordaland on January 1, 2003. The annual incidence rates for the years 1953 to 2003 were calculated. RESULTS: 100,000 among men. The annual incidence of MS increased from 1.8 per 100,000 in 1953 to 1957 to 6.0 per 100,000 in 1993 to 1997. CONCLUSIONS: Hordaland County, Norway, has changed from a low-risk to a hi During the last 25 years, the incidence of MS has been stabilized rather than increasing. Systematic longitudinal follow-up studies are essential to calculate reliable prevalence and incidence rates in MS The results suggest both both methodologic and environmental factors are essential in determining the distribution of MS.