Endoscopy is commonly accepted as the gold standard in the evaluation of neoplastic colonic disease. The procedure is used to confirm or exclude lesions detected on barium enemas, with the assumption that the endoscopist was successful in reaching the appropriate segment of the colon. We collected 18 cases, all with proved colonic neoplasm 2-8 cm in diameter that were detected by barium enema but overlooked on initial endoscopy. All of the lesions were relatively flat with little intraluminal protuberance. Histologic examination showed malignant foci in 6 of 11 tumors that were resected. In 2 of the other 7 patients, unresected lesions progressed to advanced carcinomas. This experience suggests that a repeat barium enema is indicated when endoscopy fails to detect a colonic tumor suspected on barium enema examination.