In a retrospective epidemiologic study of the period 1963 to 1987 the annual incidence of definite ulcerative proctocolitis increased from 3.3 to 14.9/10(5), and during the last 10 years the mean incidence was 13.1/10(5) population. The incidence of extensive and total colitis was fairly stable, whereas an increase of ulcerative proctitis and distal colitis was seen. The increase in incidence is probably an artefact caused by better retrieval of cases in the latter part of the study. The prevalence on 31 December 1987 was 234/10(5) inhabitants. The male to female ratio was 1.59:1, compared with 0.96:1 in the general population. The median age at diagnosis was 33 years and increased almost 7 years by the end of the study, probably caused by an increased proportion of ex-smokers, who at diagnosis were, on average, 9 years older than lifetime non-smokers. The highest age- and sex-specific incidence was found in middle-aged men, who were ex-smokers to a significantly higher extent than women. No specific birth cohort was at an increased risk, and no second incidence peak in older age groups was seen.