The incidence of Crohn's disease in western Norway was estimated in a prospective epidemiologic study during the years 1984 and 1985. The total population in the area was 807,000. Both inpatients and outpatients were included. Five hundred and fifty general practitioners and 12 hospitals participated in the study. A total of 86 patients were diagnosed, giving a mean annual incidence of 5.3 per 100,000. For patients between 15 and 19 years of age-specific incidence rate was 16.0 The M/F sex ratio was 0.9. In the same period 240 patients with ulcerative colitis were diagnosed (mean annual incidence, 14.8 per 100,000), giving a ratio of 2.8 between ulcerative colitis and Crohn's disease and an annual incidence of 20.1 per 100,000 for inflammatory bowel disease. Familial occurrence of IBD was found for 12% of the patients.