The need for colonoscopic examination of the whole colon was determined by evaluating the accuracy of a barium enema X-ray report with particular reference to the right colon. Long colonoscopies were performed on 108 patients. The reports of the barium enema X-ray examination in these patients were compared with the colonoscopy reports. Over all, the barium enema X-ray report was accurate in 47% of cases. The rate of false negative reports was 10% and that of false positive reports 43%. For lesions of the right colon, the barium enema X-ray report was accurate in only 32% of cases, with 68% false positive and 15% false negative reports. False positive reports of carcinoma or polyps result mainly from imperfect preparation. Of the eight right colonic lesions missed in the barium enema X-ray examination, five were polyps and one was carcinoma. Long colonoscopy, rather than short colonoscopy, or flexible sigmoidoscopy and barium enema X-ray examination, would be accompanied by the greatest diagnostic accuracy. Air-contrast barium enema X-ray examination and long colonoscopy together offer the best methods of diagnosis of occult lesions in the right colon.