In order to determine the frequency of tumours in patients presenting with frank rectal bleeding, a prospective study was carried out in 337 patients over the age of 40 attending our Proctology Clinic. After making a clinical diagnosis, flexible sigmoidoscopy followed by barium enema was performed. Excluding 7 digitally palpable rectal cancers, there were 30 cancers (9.1%), 34 polyps (10.3%), 7 proctocolitis (2.1%) and 25 diverticula (7.6%) detected, giving a total diagnostic yield of 29.1%. A clinical diagnosis of bleeding sources other than haemorrhoids was made in 80% of the cancers. Flexible sigmoidoscopy detected 93.3% of the cancers and 88.2% of the polyps. Barium enema diagnosed the remaining cancers and polyps but there was a false-positive rate of 3.8%. From our results, we conclude that patients with frank rectal bleeding should be screened routinely for left-colon cancer irrespective of the clinical diagnosis. The flexible sigmoidoscope is a quick and useful tool. However, barium enema should be recommended to individuals with strong clinical suspicion of rectal bleeding other than haemorrhoids.