Descriptive epidemiologic studies from a high-incidence area like Sweden indicate a stable annual incidence rate for both ulcerative colitis and Crohn's disease since the 1970s. Differences in the prevalence of ulcerative colitis in different ethnic groups in Israel, studies of perinatal events in Sweden and Canada, and cohort phenomena reported from Sweden indicate that events in the perinatal period are important in the causation of inflammatory bowel disease. Studies of the familial occurrence of inflammatory bowel disease in Denmark, Sweden, Israel, and the United States consistently show that there also is a strong genetic component in the etiology of the diseases.