Two hundred and twenty seven patients had multiple follow-up total colonoscopies after initial endoscopic polypectomy. All adenomas were removed at the initial examination ('clean colon') in those patients who had no history of colon cancer, inflammatory bowel disease or polyposis coli; when re-endoscoped within one year 56% of them were found to have further adenomas. Nine percent had adenomas over 10 mm diameter which are presumed to have been missed during the index colonoscopy. Patients with single or multiple adenomas had an equal likelihood of being found to have further lesions on a one-year follow-up colonoscopy. With an apparent chance of missing a significant-size polyp at colonoscopy of at least 10% we recommend that all patients have a follow-up colonoscopy within one year after polypectomy. 133 Patients in whom no adenomas were found at the one year colonoscopy ('negative colonoscopy') were followed up. The incidence of new adenomas occurring within four years of 'negative colonoscopy' was 35% for those with a single adenoma at the index examination and twice as high for patients with multiple adenomas. If no polyp is found at one year, we recommend that interval follow-up colonoscopies should be performed every two years if more than one adenoma was removed during the initial examination, and every three years if only one was removed.