One approach to learn about possible environmental risks in inflammatory bowel disease relates to studying its geographic pattern of occurrence. The geographic variation of inflammatory bowel disease within the United States was analyzed using the accumulated 17.5 million hospital discharges of all U.S. Medicare beneficiaries during two consecutive years. To validate the geographic pattern shown by the Medicare data, hospitalization was compared with mortality from inflammatory bowel disease among different states. Mortality and hospitalization statistics both suggested that the occurrence of inflammatory bowel disease was determined by environmental factors that had a marked geographic variation within the United States. Both Crohn's disease and ulcerative colitis appeared to be more frequent in northern parts of the United States than in southern and in urban more than rural parts. These trends were observed for men and women and for blacks and whites alike. Similar geographic patterns of Crohn's disease and ulcerative colitis suggested the influence of one or more identical risk factors for both diseases.