Current incidence figures on ulcerative colitis and Crohn's disease--presented in a recent multicenter study in Europe--are given, and differences in the frequency and clinical appearances of the two conditions are discussed. Trends in the frequency and clinical appearance of inflammatory bowel disease during the twentieth century are summarized, as well as the differences over time and from place to place. Correlations between age, sex, localization of disease and clinical symptoms are given. Risk of progression to more extensive disease in patients with proctitis is shown. Incidences of inflammatory bowel disease in childhood and among migrated ethnic groups are discussed. Survival and cancer risk among patients with ulcerative colitis and Crohn's disease are shown from long-term clinical epidemiological studies of well-defined patient groups. Trends in these important prognostic parameters over time are shown, as well as factors influencing prognosis of the diseases.