We report three patients with chronic ulcerative colitis and colonic strictures that were found to harbor advanced adenocarcinomas despite previous endoscopic examinations that had failed to demonstrate dysplasia or carcinoma. This experience calls into question the reliability of endoscopic monitoring of the subset of ulcerative colitis patients with strictures. Review of the literature supports the thesis that the management of these patients requires a high index of suspicion of malignancy, and should include timely consideration of surgery. It is suggested that evaluation of the effectiveness of endoscopic surveillance programs for patients with ulcerative colitis should consider the subset of patients with strictures as a separate cohort.