This retrospective investigation assessed the sensitivity of colonoscopy for the detection of colonic polyps seen previously at barium enema examination. Included in the study were 77 patients with 106 polypoid lesions. Films showing lesions not subsequently seen at colonoscopy were reviewed and only those lesions with a visible point of attachment, exhibiting no movement in response to filling or change in position of the patient, and confidently diagnosed as a polyp by both reviewing radiologists were included among the 106 lesions. Sixteen lesions (15%) seen radiologically were not located colonoscopically, indicating an endoscopic sensitivity of 85%. Contrary to previous reports, most of the lesions missed endoscopically were in the left colon in regions thought to have been traversed by the instrument. The 15% false-negative rate found for colonoscopy is consistent with existing reports on colonoscopic errors and is approximately the same as the false-negative error rate for radiologic detection of colonic polyps.