Fifty-six endoscopically removed malignant polyps of the colon and rectum were studied to assess criteria for adequacy of therapeutic polypectomy. Features examined were: 1) tumor grade; 2) lymphatic invasion; 3) tumor extent (head, stalk, margin); 4) sessile versus pedunculated; 5) size; and 6) type of background adenoma. Thirty-four patients underwent colon resection while the remaining 22 malignant polyps were followed for a mean of 4.5 years. Five (8.9%) malignant polyps metastasized to lymph nodes while three cases showed metachronous liver metastases. Pathologically, malignant polyps were grouped into 28 long stalk (LS), 21 short stalk (SS), and seven sessile types, with a lymph node metastatic rate of 0%, 19%, and 14%, respectively. Only one of six malignant polyps with lymphatic invasion had any lymph node metastases (16.5), while 66% of grade III cancers had lymph node metastases. In those 24 cases with tumor at or near the resection margin (17 SS and seven sessile cases), the incidence of lymph node metastases or local recurrence was 25%. The incidence of lymph node metastases or local recurrence was 0% among the 28 LS polyps and the four SS polyps with tumor limited to the head. Two of seven polypoid carcinomas (28.5%) metastasized; however, both had positive resection margins. There was no difference in size between metastasizing and nonmetastasizing malignant polyps. Of the 36 cases where histological criteria indicated polypectomy inadequate, the incidence of lymph node metastases or local recurrence was 17%. There were no metastases or recurrences where polypectomy was considered histologically adequate. LS polyps may be treated by polypectomy alone, except in those cases with grade III cancer, lymphatic invasion, or tumor at the resection margin. SS polyps with cancer limited to the head may be treated similarly to LS polyps, while all other SS polyps and sessile polyps should undergo resection postpolypectomy.