From the information we have provided in the first three parts of this series of articles, and based on histopathologic study of specimens available for us, we conclude that what was called for more than 200 years "colonic adenoma," is not really adenoma, namely, a benign neoplasm of glandular epithelium, but is carcinoma in situ, obviously a well-differentiated one. In sum and in short, we believe that the term carcinoma in situ is a straightforward and easy-to-understand term, and that the criteria for diagnosis of it are lucid and repeatable. Most important, this term reflects the true nature of these lesions and explains why these lesions when discovered endoscopically should be removed completely.