We have previously found an increase in prevalence and incidence of multiple sclerosis (MS) in the county of Hordaland, Western Norway. This study shows that the increase in incidence over a 30-year period is due to an increase of remittent and remittent/progressive MS. The incidence of the chronic progressive form of MS has remained stable during the same period. The two subgroups of MS also vary in symptomatology and age at onset. This suggests that MS has two different forms which behave differently when considered epidemiologically and clinically. The remitting and progressive forms may therefore each have a different etiology.