Summary: The uterine cervixes of 58 patients with cervical adenocarcinoma (seven in situ, 16 microinvasive, and 35 frank invasive) were studied along with 210 cervixes that did not show any disease. Squamous metaplasia was confirmed in surface and cleft epithelium, but not in the deepest area of the gland-bearing portion of the cervix. Ectopic endometrial glands were observed in two of the 210 cases. The maximum extent of in situ and microinvasive endocervical adenocarcinoma was located in the area of the primary cleft; and that of endometrioid adenocarcinoma, in the deepest area of the gland-bearing portion. It is suggested that endocervical adenocarcinoma originates from the primary cleft and that neoplastic epithelium replaces normal gland epithelium starting from the shallow part of the cleft and spreading to the deep part of the cervical canal. Endometrioid adenocarcinoma is considered to originate mainly from the cells of the deepest tunnels, as well as from ectopic endometrial glands; its initial growth occurring by means of invasive proliferation into the stroma.