Colonoscopic removal of large, sessile polyps is difficult, but can be successfully carried out by experienced endoscopists. "Piecemeal" resection with an electrocautery snare was performed at our institution in 108 patients with 132 such lesions. The mean size of the unresected polyps was 3.0 cm. Complications occurred in 3.0% of polypectomies (3.8% of patients), with bleeding necessitating transfusion in 2.3% of polypectomies (2.8% of patients), and microperforation (probable) in the remainder. No patient required emergency surgery due to a complication. In 65 patients (60%), colonoscopic resection and follow-up alone was carried out. Of these, adenomas recurred/persisted in 28%, most of which were successfully re-resected. Nearly half of all recurrent polyps occurred after at least one negative intervening examination. Carcinoma later appeared in 17% of the recurrences despite apparent initial complete resection of a previously benign polyp. Cure was ultimately achieved in 88% of endoscopically managed patients. Surgical resection was required in 27% of patients, mostly following the initial polypectomy when invasive carcinoma was found in the specimen. No residual tumor was later found in 41% of the colon specimens from these patients. Ninety-one percent of cancers were favorable stage, whether discovered early or late. Follow-up colonoscopy was achieved in 77% of patients over an average of 3.7 years. Metachronous polyps were excised in 52 patients (63%) and metachronous carcinoma was diagnosed in 3 patients (3.6%). An aggressive regimen of surveillance colonoscopy is warranted in these patients to detect and manage local recurrences and to remove subsequent adenomas. Endoscopic resection of large sessile adenomas can be safe and effective.