Adenocarcinoma in situ (AIS) and microinvasive adenocarcinoma of the uterine cervix and normal endocervical columnar epithelium were studied by cytology, morphometry and electron microscopy to identify differentiating features and to ascertain the cellular origin of cervical adenocarcinoma. Smears from AIS showed the characteristic cytology, consisting of glandular rosettes, palisading and crowded sheets; most nuclei had a relatively uniform oval shape. Smears from microinvasive adenocarcinoma showed more crowded sheets, with enlarged, round and irregular-shaped nuclei and prominent oval nucleoli. These nuclear features were confirmed by the morphometric results. Ultrastructurally, reserve cells in the normal tissues contained tonofibers and secretory granules and showed squamous and adenomatous features. The ultrastructural features of microinvasive adenocarcinoma were similar to those of well-differentiated invasive adenocarcinoma. The cells from both contained tonofibers and secretory granules. These findings suggested that the reserve cell is the cell of origin for cervical adenocarcinoma.