To determine the incidence and clinical pattern of Crohn's Disease in a defined area in Germany, a prospective, population-based study was carried out from 1980 to 1984 and again from 1991 to 1995. All patients newly diagnosed with Crohn's disease within the respective study period who were resident in the study area were included in the study. The results from both study periods were then compared to detect time trends. Altogether 288 (156 and 132, respectively) incident cases were identified yielding an almost unchanged incidence over the years (1980-84: 4.9/10(5); 1991-95 5.2/10(5)). While the peak of incidence is still in the 15-24-year-old group, 1 out of 5 incident patients is now age 50 years and older. Median age at onset of symptoms increased to 30 years (20 years in the former period). Time from onset of symptoms was reduced from a median of 20 months in the 1980s to 5 months. Symptoms did not change significantly, although there seems to be less complicated disease recently. Distal migration of the inflammation in the intestinal tract was observed with significantly more involvement of the sigmoid and rectum in the recent period.