Background: The nature of so-called adenoma of the colon is a controversial and confusing issue for all students of this subject. Understanding the exact nature of a disease, however, is a requisite to appropriate treatment. Objectives: To clarify the nature of adenoma of the colon. Patients/Methods: Series of articles including (1) review of historical literature; (2) review of current literature; (3) studies of histopathologic features in examples of so-called colonic adenoma; (4) integration based on comparison with neoplasms of the skin and breast. Results: Part IV—A histopathologic comparison of squamous cell carcinoma in situ of the skin, ductal carcinoma in situ of the breast and so-called colonic adenoma was conducted. The comparison further reinforces the notion that so-called colonic adenoma is really carcinoma in situ and indicates that carcinoma in situ is a unifying concept that can be applied to every epithelium. Conclusions: The so-called colonic adenoma is not adenoma, namely, a benign neoplasm of glandular epithelium, but is carcinoma in situ.