The incidence of ulcerative colitis was estimated during the period 1962 to 1987 in the county of Copenhagen. Within this area of approximately 550,000 inhabitants, 1161 patients were diagnosed. The mean annual incidence for the period was 8.1 per 10(5) inhabitants. There were significant increases in incidence in the early 1970s and in the early 1980s, both of which were followed by significant decreases. A bimodal age distribution was found in men, with incidence peaks in young adult life and late in life. In women a peak incidence was found in the young adult age group. The interval from onset of symptoms to diagnosis in terms of years remained unchanged over the period, with a median of 1 year (range, 0-37 years). The extent of disease at diagnosis was total colon in 18% of the patients, a substantial part of the colon in 36%, and distal colon in 44%--with no changes during the study period. The distribution of different localizations did not differ among age groups except for a tendency towards more extensive disease in young patients, below 20 years of age. The disease activity in the 1st year was fulminant in 9.1% of the cases, moderate to high in 70.7%, and low in 20.2%, with a tendency towards higher activity in patients diagnosed late in the study period. The prevalence of ulcerative colitis increased steadily during the study period, reaching a value at the end of the study (31 December 1987) of 161 per 10(5) inhabitants.