Colonic villous adenoma is a type of adenoma and is a benign tumor. High-grade intraepithelial neoplasia is precancerous and includes severe heterogeneous hyperplasia and carcinoma in situ. Adenomas are common benign tumors of the colon with some risk of malignancy and can be classified as tubular adenomas, villous adenomas, and mixed adenomas. Choriocapillary adenomas have a higher risk of malignancy than the other two. High-grade intraepithelial neoplasia is a precancerous lesion that includes severe heterogeneous hyperplasia in which the cumulative number of heterogeneous hyperplastic cells is >2/3 of the mucosa and carcinoma in situ in which the cumulative number of heterogeneous hyperplasia cells is all over the mucosa, but has not broken through the basement membrane. High-grade intraepithelial neoplasia should be treated with endoscopic mucosal resection or dissection as soon as possible. Patients are advised to consult their specialists and follow medical advice to avoid progression of precancerous lesions.