The use of the colonoscope to evaluate the cause of rectal bleeding in a series of 2200 patients has been reviewed. The most common cause of bleeding in this series was found to be neoplastic polyps, which were present in 723 patients (32 per cent). Colonic carcinoma was detected as the source of the bleeding in 425 cases (19 per cent). Although a large number of barium enema films were false negatives, the patients reviewed are a highly select group. It is still believed that barium enema studies and colonoscopy are complementary rather than competitive procedures. Their continued combined use greatly enhances diagnostic accuracy. Various other causes of colonic bleeding, including inflammatory bowel disease, arteriovenous malformations, endometriosis, ovarian carcinoma, ischemic colitis, and radiation colitis, have been discussed and their endoscopic appearance described. Of particular significance is the coexistence of internal hemorrhoids or diverticular disease and neoplastic colonic lesions. Barium enema films and sigmoidoscopy have been frequently described as the twin pillars of diagnosis in the detection of colonic pathology. Colonoscopy, as the third pillar of diagnosis, should be an integral part of the evaluation of patients with rectal bleeding.