We have examined the time trends in multiple sclerosis incidence over the past four decades from 1950 to 1991 in Møre and Romsdal County, Norway. Incidence rates by year of birth, year of onset and year of diagnosis according to sex, age and clinical course were studied. All patients with multiple sclerosis diagnosed by August 1992 were included, giving a total of 419 patients (171 men, 248 women) with onset of multiple sclerosis in the period 1950-91. Mean age at onset was 33.2 years, and mean age at diagnosis was 39.2 years. The incidence rate by year of onset increased from 2.87 per 100 000 in the period 1950-54 (men 3.06, women 2.67) to 5.57 per 100 000 in 1985-91 (men 3.75, women 7.94) (P<0.001). The incidence was particularly high in the period from 1975 to 1985. Major period- or cohort-effects in incidence, apart from the general increasing trend, could not be identified by an age-period-cohort analysis.