Adenocarcinoma in situ (ACIS) of the cervix is rare and is frequently overlooked. To characterize this disease more fully, 21 cases were studied. All except two patients presented with abnormal smears. The distribution of ACIS was focal in two cases, multicentric in three, and diffuse and continuous in 15 (in one case it was unknown). The depth of crypt involvement varied from 0.5 to 4 mm and the volume was estimated to range from 0.25 to 1,500 mm3. ACIS should and can be distinguished from early ("microinvasive") adenocarcinoma in most cases by its limitation to the glandular field, by the constant admixture of neoplastic and normal glands, and by the lack of stromal response. Invasive adenocarcinoma cannot be excluded by target biopsy, the diagnosis of ACIS requiring conization. If the surgical margins are disease free, conization alone may be adequate therapy.