The results of fibreendoscopy of the colon are described in 255 consecutive examinations. Of the examinations, 26.5% resulted in a diagnosis being made solely by endoscopy. This included 17 cases of carcinoma, 15 patients with polyps, and 25 patients with inflammatory bowel disease. The most common reason for referral was undiagnosed rectal bleeding (75 cases) and endoscopy alone was successful in diagnosing the probable or definitive source of bleeding in 50% of the cases referred. It is not yet clear, however, to what extent this figure may reflect the inadequacy of conventional radiology.A definitive radiological diagnosis was refuted in 11 patients and an unnecessary laparotomy avoided in seven of these. There is no doubt that fibreoptic colonoscopy increases diagnostic accuracy in large bowel disease and is especially helpful in cases where radiology is either negative or equivocal.