Colonoscopic examination of the entire colon was performed on 146 patients for radiographically suspected benign polypoid disease. Of thirty-six patients who did not have a neoplastic lesion at the suspected site, seven (19 per cent) had unsuspected small benign polypoid adenomas elsewhere in the colon. Of the remaining 110 patients who had a neoplastic lesion at the radiographically suspected site, 17 lesions (15 per cent) were either adenocarcinomas or neoplastic polyps with invasive carcinoma. One hundred twenty-eight additional unsuspected neoplastic polyps were found in 62 of the 110 patients (56 per cent). Six of the additional neoplastic lesions were either adenocarcinomas or polyps with invasive carcinoma. Four of these malignant lesions were in patients who had a benign polyp at the radiographically suspected site. Suspected colonic polypoid disease should be evaluated colonoscopically despite radiographic evidence of benignity. Colonoscopic evaluation in colonic polypoid disease should include examination of the entire colon with pathologic documentation of all polypoid lesions encountered.