In order to determine the feasibility, yield, and impact of routine total colonoscopy on the management of large-bowel cancer, 157 cancer patients underwent 175 colonoscopic procedures; 13.6 per cent of the cancers had been missed on double-contrast barium enema examination. Among 92 patients undergoing perioperative colonoscopy, the lesion was reached in 90 per cent and the cecum in 60 per cent; 7.6 per cent demonstrated synchronous cancers, all curable, and all missed on barium-enema examination. Seventy-eight patients underwent colonoscopy at an average of 3.7 years after treatment of the index cancer; 7.7 per cent demonstrated metachronous cancers, all curable, two-thirds of which were missed on barium-enema examination. Benign polyps were noted in 62 per cent of the patients studied; 77 per cent of those polyps, 1 cm or greater in size, were missed on barium-enema examination. Polyps were found proximal to the cancer in 60 per cent of the patients with polyps. Approximately 85 per cent of those with multiple cancers demonstrated benign polyps. Preoperative total colonoscopy with periodic postoperative colonoscopy at an interval of three to five years are essential in the reliable detection of synchronous cancers and for the detection of metachronous cancers at an earlier, more favorable stage.