In certain clinical situations, fiberoptic colonoscopy has proved most useful and effective as a diagnostic and therapeutic tool. Colonscopy is indicated to remove polyps when feasible and to rule out the presence of toher polyps or tumors undetected by barium enema. Patients with cancer of the colon should have preoperative colonoscopy to rule out the presence of undetected polyps or metachronous cancer. Follow-up examinations are important to insure against residual cancer or recurrence after colonscopic removal of sessile, premalianant, or malignant polypoid lesions, and to evaluate certain patients who have increased risk of polyp or tumor formation. Colonscopy usually can explain persistent, localized abnormalities detected by barium enema (ie, "filling defects") and provide a definitive diagnosis in cases of unexplained rectal bleeding or diarrhea despite negative sigmoidoscopic and barium studies. In experienced hands, this technic is safe, confortable, and effective in avoiding laparotomy to remove clinically significant polyps and in providing definitive diagnosis in many clinical situations.