The incidence of ulcerative colitis and of Crohn's disease in the County of Copenhagen was estimated during the years 1962 to 1978. A total of 909 patients were diagnosed in this area with approximately 500,000 inhabitants. The mean incidence of ulcerative colitis was 8.1 per 10(5) inhabitants. This incidence was constant during the period for women but rose significantly around 1970 for men. The rise in male incidence is due to a rise in the incidence late in life. The prevalence of ulcerative colitis as of December 31, 1978 was 117 per 10(5) inhabitants. The incidence of Crohn's disease was increasing during the period for both sexes with a mean value for the period 1970 to 1978 to 2.7 per 10(5) inhabitants. The prevalence of Crohn's disease as of December 31, 1978 was 34 per 10(5) inhabitants.