Fifty-four endoscopically missed colonic polypoid lesions reveal the limitations of colonic endoscopy. Colonoscopy failed to identify 31 of the lesions, whereas 24 lesions were initially missed by proctosigmoidoscopy. One polypoid tumor of the rectosigmoid junction was missed by both. Most undetected lesions ranged from 0.5 to 1.5 cm. Endoscopic and radiologic diagnostic techniques are clearly complementary. Their combined diagnostic accuracy exceeds that of either technique alone. If results of the two methods conflict, one or both should be repeated. With a combined approach, completely missed lesions can be kept at a minimum.