Pathobiology of dysplasia in chronic inflammatory bowel disease: Current recommendations for surveillance of dysplasia. Patients with ulcerative colitis and Crohn's disease bear an about 10- and 4-fold increased risk, respectively, for developing colorectal carcinoma. Apart from typical locations of colorectal carcinoma in the sigmoid colon and rectum other locations were also often observed, e. g. right hemicolon or multifocal distribution. Histologically colorectal neoplasms frequently present as mucinous adenocarcinoma (signet-ring cell carcinoma). The risk for neoplasm depends on extension, severity, duration and therapeutic responsiveness of chronic colonic inflammation, and it seems pathogenetically to be similar in ulcerative colitis and Crohn's disease. Colorectal carcinoma in inflammatory bowel disease arises from epithelial dysplasia. Since there are no reliable biological markers available to date, surveillance-programs continue to rely on the discovery of dysplasia (unequivocal intraepithelial neoplasia). Detection of dysplasia by colonoscopy achieves 70-85 % sensitivity.Endoscopic surveillance should start after 8 years of disease's duration in pancolitis, after 10-12 years in left- sided colitis and after 12 years in Crohn's disease of the colon, with regular intervals every 1-2 years. 3-5 biopsies should be done every 10 cm from mucosa free of inflammation. Additionally, every fine or discrete alteration of the mucosal surface should be recorded. Multiple biosies should also be taken from such minimal lesions as well as from more macroscopically suspicious areas like plaques, nodular lesions or stenosis. The clinical consequence of a positive screening for dysplasia is colectomy because of an assumed risk of cancer of about 40-70 %. Dysplasia in macroscopically suspect areas bear the highest risk of cancer (non-adenoma like dysplasia), followed by multiple high-grade lesions without a macroscopic lesion, and multiple low-grade dysplasias. Detection of single dysplastic lesions in flat mucosa should be followed by a control endoscopy after 2-6 months, and if dysplasia is seen again, colectomy is recommended.