The issue of whether patients who have invasive carcinoma in an otherwise benign polyp should undergo surgical resection of that bowel segment is controversial. We examined the records of 83 patients with such polyps that were removed endoscopically between 1978 and 1981. After exclusion of 18 patients from the study for various reasons, our study group consisted of 65 patients who had undergone complete endoscopic removal of 69 polyps that contained a malignant process. Carcinoma in situ was found in 34 polyps, and 3 patients with such polyps had a recurrent malignant lesion of the same degree. These recurrent tumors were treated successfully by aggressive endoscopic removal. Seventeen polyps that contained invasive carcinoma were treated endoscopically without resection, and two patients in this group subsequently had recurrent carcinoma at the site of the original polyp. These recurrent lesions were resected, and neither patient had evidence of nodal metastasis. Of the 18 patients who underwent immediate resection of invasive carcinoma, 2 had residual carcinoma at the time of the resection, although no lymph node metastasis was found. We conclude that colonic polyps with carcinoma in situ can be treated safely with complete endoscopic removal. Invasive carcinoma in a polyp can be treated safely with complete polypectomy, and immediate resection may not be necessary. This group of patients, however, should undergo thorough follow-up studies and periodic endoscopic reexamination.