An epidemiologic study of the incidence of Crohn's disease during the 25-year period 1963-1987 within a geographically well defined area has been performed. The mean annual incidence during the 25 years was 6.1/10(5) inhabitants. During the first quinquennium the incidence was 4.3/10(5) inhabitants, but during the next four quinquennia the incidence was very stable around 6.6/10(5) inhabitants. The point prevalence on 31 December 1987 was 146/10(5), with a male to female ratio of 1:1.15. An analysis of birth cohorts could not reveal any cohort more prone to develop the disease. Of the 246 incidence cases 5 patients had died of Crohn's disease. The percentage of small-bowel disease only was very stable during the whole study period. The proportion of large-bowel disease decreased only slightly, and combined small- and large-bowel disease increased somewhat towards the end of the study period, probably because of better diagnostic methods.