Localized high-grade intraepithelial neoplasia is present in the colon, i.e., heterogeneous hyperplasia of cells involving two-thirds to all of the mucosal glandular epithelium. Heteroplasia, also known as atypical hyperplasia, is a condition in which the proliferating cells differ from normal cells in terms of morphology, arrangement, and nuclear pattern. Heteroplasia involving <1/3 layer of colonic mucosal glandular epithelium is mild heteroplasia, 1/3~2/3 is moderate heteroplasia, >2/3 but not involving the whole layer is not severe heteroplasia. If the heterogeneous proliferation cells involve the whole mucous membrane, but do not break through the basement membrane to infiltrate into the deeper layers, it is carcinoma in situ. Heteroplasia (atypical hyperplasia) and carcinoma in situ are precancerous lesions. The progression from heterogeneous hyperplasia to carcinoma in situ is called intraepithelial neoplasia. Mild and moderate ectodermal hyperplasia is called low-grade intraepithelial neoplasia, while severe ectodermal hyperplasia and carcinoma in situ are high-grade intraepithelial neoplasia. High-grade intraepithelial neoplasia of the colon is associated with a high-fat diet and intestinal inflammation. It is recommended that the patient should consult the doctor in time and undergo endoscopic mucosal resection or endoscopic mucosal dissection if necessary, and then follow the doctor’s instructions for follow-up treatment and regular checkups.