There is no definitive statement on how big a rectal adenoma can be before it becomes cancerous, and the rate of adenoma cancer correlates with the type of adenoma pathology and has little to do with size. Adenomas are a group of polyps, which are not only the most common, but are considered precancerous lesions of colon and rectal cancer. Adenomas are initially mucosal elevations, and the common pathologic types are tubular adenomas, choriocapillary tubular adenomas, and choriocapillary adenomas. Among them, the cancer rate of tubular adenomas is not high, and the cancer rate of choriocapillary adenomas can be as high as 55% to 75%. The cancer rate of rectal adenomas is not directly related to tumor size. The development of rectal cancer from adenomatous polyps is a very slow process. Waiting for it to undergo a series of genetic changes, the process can take up to 5 to 10 years. If rectal polyps are detected, they should be followed up regularly and closely observed.