An epidemiologic study of inflammatory bowel disease was conducted in Regio Leiden, the Netherlands, between 1979 and 1983. Archives of endoscopy, radiology, pathology, and specialist letters were reviewed for suspected patients with inflammatory bowel disease, together with a survey of all general practitioners to verify completeness of data. One thousand forty patients were identified and each diagnosis was reviewed. Two hundred ten patients had Crohn's disease and 257 had ulcerative colitis. Of the other 573 patients, the largest proportion (21%) had incomplete data for disease classification. Others had irritable bowel syndrome, diverticulitis, or ischemic or irradiation colitis; some were nonresident patients with inflammatory bowel disease treated within the region and others were out of the period for inclusion in this investigation. The incidence of Crohn's disease was 3.9 per 10(5) per year and the period prevalence was 48 per 10(5). The sex-specific incidence was similar, although the disease was significantly more common in women aged 20-29 yr. The prevalence in the city municipalities of Leiden and Alphen on the Rijn (63 per 10(5)) was similar but significantly greater than in suburban (39 per 10(5)) or agarian areas (40 per 10(5)). This may be partially due to urban density but not to differences in water supply. The lack of cases in the migrant population almost reaches significant levels, but studies in locations with a higher migrant population may clarify the issue.