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 III. A histopathologic reassessment of 30 cases of previously diagnosed colonic adenoma was undertaken. We found that all lesions showed histopathologic features fulfilling the criteria for diagnosis of carcinoma in situ, namely, cytologic features of carcinoma such as crowding, stratification, and overlapping of large hyperchromatic nuclei, loss of maturation, single cell necrosis, increased number of mitotic figures, with all neoplastic cells confined in the colonic epithelium. Conclusions: The so-called colonic adenoma is not adenoma, namely, a benign neoplasm of glandular epithelium, but carcinoma in situ, obviously a well-differentiated one.