Between 1984 and 1987, we reviewed all large (greater than or equal to 3.0 cm) colorectal polyps to determine the efficacy of colonoscopic polypectomy from both an oncologic and technical viewpoint. Forty-eight polyps greater than or equal to 3.0 cm were identified in 46 patients. Twenty polyps were entirely benign, 20 polyps contained noninvasive carcinoma, and invasive carcinoma was present in eight polyps. Four of the invasive cancers were associated with residual adenoma; the remaining four were polypoid carcinomas. Among the eight cases of invasive carcinoma, four had tumors that did not extend through the submucosa. Invasive cancer was more prevalent in left-side sessile lesions but was absent in all 10 right-sided polyps. Thirty-two polyps were removed by colonoscopic polypectomy. Four patients required colectomy after polypectomy for the following reasons: incomplete excision (N = 1), presence of invasive carcinoma at the resection margin (N = 1), and inability to define the level of carcinoma on pathologic examination (N = 2). Two polyps with cancer confined to the submucosa were successfully excised colonoscopically. Complications of polypectomy included three cases of minor hemorrhage. Sixteen polyps (the majority located in the right colon) were removed by primary surgical colectomy. We conclude that colonoscopic polypectomy is oncologically and technically successful for most large colorectal polyps. A minority of large polyps require colectomy because of incomplete removal or the presence of invasive cancer that is not curable with colonoscopic excision.