Background: Differentiating primary glandular from high-grade squamous intraepithelial lesions (HSIL) that involve endocervical glands is not an uncommon diagnostic problem in liquid-based gynecological cytology. Squamous and atypical glandular cell lesions may show similar cytomorphologic features. The aim of this study was to evaluate the use of p63 as a marker of basal and/or squamous cell derivation in this differential diagnosis., Methods: Of 59,257 liquid-based cervicovaginal specimens collected over a 3-year period, 149 were diagnosed as atypical glandular cells of uncertain significance (AGUS) or adenocarcinoma and had histological follow-up. Ten cases (8AGUS and 2 adenocarcinomas) were proven to be high-grade dysplasia on cervical biopsies and the remaining cases represented glandular pathology. Slides from discrepant cases were stained with p63 antibody. In addition, the authors stained 25 control cases (10 adenocarcinomas, 10 HSIL, and 5 negative cervicovaginal specimens)., Results: In all 10 discrepant cases, the abnormal groups originally interpreted as glandular in origin showed a homogeneous strong nuclear staining for p63 that indicated their squamous origin. Nuclei of isolated HSIL cells and basal cells from atrophic smears were also positive for p63. Benign and malignant glandular cells were uniformly negative. Isolated metaplastic, intermediate, and superficial squamous cells were likewise negative for this antibody., Conclusions: p63 is a useful immunocytochemical marker for differentiating primary glandular pathology from HSIL in cervicovaginal specimens. It also detects isolated HSIL cells ("litigation cells"). This antibody is not expressed in AGUS, adenocarcinoma, or normal glandular cells. p63 stains basal cells and may be a diagnostic pitfall in atrophic cervicovaginal specimens., ((c) 2006 American Cancer Society.)