Carcinoma of the large bowel arises almost exclusively in neoplastic polyps, which are present in 5-10% of the population over age 40. These polyps may be of three histologic types: (a) tubular adenoma (adenomatous polyp), (b) tubulovillous adenoma (villoglandular polyp), and (c) villous adenoma. The tendency for malignant change is greatest in the villous adenoma (41%) and in tubulovillous (villoglandular polyps--23%). In tubular adenomas (simple adenomatous polyps), the incidence of malignancy is 5% over all. The tendency toward malignant change is determined by histologic type, size, and degree of atypicality of adenoma. Early detection and complete removal of neoplastic polyps of the colon are essential to the prevention of cancer of the large bowel, and this may usually be done in most cases with the colonoscope.