Suggestions were made in the past as to how patients with ulcerative colitis who are at risk of developing cancer should be managed. Ba enema and sigmoidoscopy are used most frequently for this purpose. Colonoscopy with multiple biopsies throughout the colon may detect early cancer. The expertise of the endoscopist is important in such an examination, since he must determine the areas of the colon where biopsy will be most fruitful. Although the colonoscopist in the present case performed 350-400 procedures and is recognized as an expert, he failed to detect the cancer. Specimens taken from 19 patients with ulcerative colitis were examined by Cook et al., and 5 of 26 carcinomas could not be detected on gross examination, although the diagnosis was proved microscopically. Total colectomy should be considered in all patients with widespread colitis that has been active for longer than 10 yr. Colonoscopy is an excellent means of evaluating the extent of the disease and degree of activity, but it is not completely reliable in the detection of cancer.