Of 323 patients with colorectal polyps that either were larger than 1 cm in diameter or were 1 cm or smaller but had been subjected to biopsy or excision, 20 (6%) had subsequent development of a colorectal carcinoma, an incidence rate of 4.6 per 1,000 person-years of follow-up. These 20 cases of cancer compared with 7.38 cases expected; thus, the risk of colorectal carcinoma in these patients was 2.7 times that in the general population (P less than 0.001). Although risks were greater for certain subgroups of patients, the differences detected were usually modest. We conclude that patients with large colorectal polyps (more than 1 cm in diameter) should be closely followed up after treatment of the initial polyp, regardless of the size, site, or histologic type of the polyp, the age or sex of the patient, or the type of initial treatment (excision or fulguration). Those patients with more than one polyp, especially if more than one segment of the colon is involved, need special attention. Examination of the entire colon is important because the site of development of carcinoma may be distant from the site of the initial polyp.