Background: The incidence of ulcerative colitis (UC) has been difficult to interpret because prospective studies have only been performed during the past 3 decades. Geographic variations may therefore be due to differences in study design.
Method: From 1 January 1990 to 31 December 1993 all new cases of UC in four counties in southeastern Norway were prospectively registered. Cases diagnosed as indeterminate colitis (IND) when endoscopy and histopathology were inconclusive or diverged with regard to diagnosis of UC or Crohn's disease (CD) were also included in the study.
Results: A total of 525 cases of UC and 93 cases of IND yielded an mean annual incidence of 13.6/10(5) and 2.4/10(5), respectively. There were differences in incidence between counties, and a peak of 21.5/10(5) in the annual incidence was found for the age group 25 to 34 years in UC. The distribution was about equal for each of the groups proctitis and left-sided and extensive colitis. The time interval from onset of symptoms to diagnosis was 4 months.
Conclusion: In this study one of the highest incidences of UC in the world has been found. The classification 'indeterminate colitis' seems reasonable to use in some of the cases to prevent misclassification at the initial stage of diagnosis.