OBJECTIVE: To elucidate the difficulty of cytologically differentiating high grade squamous intraepithelial lesion (SIL) with extension into the endocervical clefts from adenocarcinoma in situ (AIS). Criteria for the cytologic diagnosis of AIS have been delineated. However, it may sometimes be difficult to differentiate between AIS and carcinoma in situ (CIS). STUDY DESIGN: We reviewed cervical smears initially diagnosed as glandular intraepithelial neoplasia (GIN) with or without associated SIL CIN; in total, slides from 10 patients were studied. The final diagnosis in two cases was SIL plus GIN, in three cases high grade SIL (HSIL) (CIN 3) plus tubal metaplasia and in five cases HSIL without GIN. RESULTS: The cervical smears from the last five cases showed, besides features diagnostic of HSIL, the presence of large, crowded sheets with feathering, consisting of fusiform cells with an oval, bare, hyperchromatic nucleus, reminiscent of AIS. At one end of these sheets, normal endocervical cells were sometimes present. In all these cases the cone biopsy revealed HSIL with extension into the endocervical clefts. CONCLUSION: In retrospect, differential diagnosis with AIS is possible in most cases if diagnostic criteria are strictly applied. Indeed, contrary to AIS, feathering is often restricted to one end of the crowded sheets. Moreover, none of these crowded sheets contains glandular openings, and strips and rosettes with pseudostratification are absent.