Background:Diagnosis of endocervical glandular involvement by high-grade squamous intraepithelial lesion (HSIL-EGI) on Papanicolaou (Pap) smears can affect the clinical management of patients.Aim:The cytological criteria for the diagnosis of HSIL-EGI are described and the accuracy of this diagnosis was investigated.Materials and Methods:Seventeen patients diagnosed with HSIL-EGI and 40 patients with diagnosis of HSIL on Pap smears with follow-up cone or loop electrocautery excision procedure (LEEP) biopsies were included in the study. The following criteria were evaluated for the cytological diagnosis of HSIL-EGI: atypical cells with definite features of HSIL, three-dimensional atypical squamous cell clusters (TDCs) with attached benign endocervical epithelium, finger-like TDCs covered with intact epithelium on most sides, which represent the finger-like invaginations of the endocervical glandular area involved by HSIL, and the absence of cytological findings of in situ adenocarcinoma of the cervix.Results:On subsequent histopathological evaluation, 16 of 17 (94.1%) patients with a cytological diagnosis of HSIL-EGI and 17 of 40 (42.5%) patients with HSIL exhibited endocervical glandular involvement (P < 0.001, sensitivity: 48.5%, specificity: 95.8%, positive predictive value: 94.1%, negative predictive value: 57.5% and accuracy: 68.4%).Conclusion:Diagnosis of HSIL-EGI may be possible on Pap smears with a high positive predictive value and specificity but low sensitivity, possibly due to cytological sampling limitations. To clarify the results of the present study, more extensive studies with a particular emphasis on the sampling of the endocervical glandular area for cytological evaluation of the cervix are needed.