The incidence and prevalence of Crohn's disease (CD) and ulcerative colitis (UC) were investigated retrospectively for 1970-1980 and prospectively for 1981-1984 in the predominantly rural county of Tübingen (FRG). Eight hundred and twenty-eight patients with CD and 376 patients with UC were detected. Point prevalence at the end of 1984 was 54.6 for CD and 24.8 for UC, respectively. The occurrence of UC was stable during the period of investigation. The annual incidence of CD rose during the end of the seventies and afterwards reached a plateau of about 4 new cases per 100,000 inhabitants per annum. The prevalence of IBD was markedly greater in the cities than in the rural areas of the county.