High-grade intraepithelial neoplasia of the rectum, it is important to first understand the concept of high-grade intraepithelial neoplasia, which is defined to include both structural and cytologic abnormalities. Structural abnormalities are defined as disorganized epithelial arrangement and acute loss of normal cells. Cytologic abnormalities are defined as irregularity of the nucleus, deep staining, elevated nuclear plasma ratio, and increased nuclear fission activity. The concept of intraepithelial neoplasia places more emphasis on the process of tumor evolution, which can be divided into low-grade intraepithelial neoplasia and high-grade intraepithelial neoplasia according to the process of evolution. High-grade intraepithelial neoplasia refers to structural and cytological abnormalities extending to the upper part of the epithelium or even to the whole epithelium, which is equivalent to the original severe heterogeneous hyperplasia or carcinoma in situ. Therefore, strictly speaking, high-grade intraepithelial neoplasia of the rectum is an early stage tumor.