Reevaluation of 178 patients treated for multiple colonic polyposis by abdominal colectomy and restoration of bowel continuity confirmed that patients with both rectal and colonic polyps are at substantial risk of having rectal cancer develop postoperatively. Rectal cancer has not occurred in any of 35 patients who had no rectal polyps preoperatively. However, 46 (32%) of 143 patients with multiple colorectal polyposis have had cancer develop during a median follow-up of nearly 20 years. Multivariate analysis showed a highly significant association between the number of rectal polyps present preoperatively and decreased survivorship free of rectal cancer (P less than .001), and a strong correlation between the presence of cancer in the resected colon and subsequent development of rectal carcinoma (P less than .01). No correlation could be established between low anastomosis and prevention of rectal carcinoma. The risk of cancer developing in the retained segment of large bowel can be established only by extended postoperative observation.