Two cases of cervical intraepithelial neoplasia (CIN) with superficial extension to the endometrium are presented, and the English literature is reviewed. The concept of the radial growth phase of CIN is presented and discussed. The radial growth phase may precede the vertical or invasive phase of neoplastic growth. The possibility of endometrial involvement in CIN points to the potential hazard of not evaluating the endocervix and endometrium before treatment of cervical CIN.