Ulcerative colitis is a highly premalignant condition, particularly when there is total colonic involvement with a disease duration of 10 or more years. Screening of appropriately selected individuals at risk for colon cancer by periodic surveillance for colonic dysplasia appears to be useful in detecting those lesions at an early treatable stage. The histology of dysplasia, a term reserved for epithelial changes that are unequivocally neoplastic, is still not completely defined, and requires a pathologist familiar with the lesion for its proper interpretation. There is a mildly increased risk of cancer of the colon and intestine in Crohn's disease, but screening of these patients does not appear to be clinically warranted.