Epidemiologic data concerning the risk for colorectal cancer in ulcerative colitis are based on a variety of large and well-conducted cohort studies. Guidelines for tumor prevention have been published by the German Gastroenterology Society (DGVS) in 2004. By contrast, the risk constellations in Crohn's disease are less clear and guidelines are lacking. Based on a recent literature search (PubMed March 2007), the published data predominantly expressed as relative risk and/or standardized incidence/mortality ratio are critically reviewed with recommendations for tumor prevention strategies based on individual risk constellations.