p16 is a tumor suppressor gene that plays a central role in the regulation of the cell cycle. In squamous cervical cancers, overexpression of p16 is induced by HPV and associated with the carcinogenesis of cervical epithelia. The aim of this study was to determine whether immunostaining of p16 is useful in detecting adenocarcinomas of the cervix uteri in histologic and cytologic routine specimens. A total of 45 surgical specimens, including 18 cases of invasive carcinoma, 8 cases of adenocarcinoma in situ, 4 cases of endocervical glandular atypia (cervical glandular intraepithelial neoplasia), and 15 reactive lesions of the endocervical glands were immunostained using a specific anti-human p16 monoclonal antibody (clone E6H4, mtm laboratories AG, Heidelberg, Germany). Furthermore, immunocytochemical analysis was performed on 10 preoperative ThinPrep cytologic samples with abnormal glandular cells and compared with the human papillomavirus status as assessed with the Hybrid Capture II test. p16 was detected immunohistochemically in all 26 cases of adenocarcinoma of the cervix uteri, including 18 invasive and 8 in situ carcinomas. Only a focal expression was evidenced in the four specimens with endocervical glandular atypia, and no reaction was found in reactive lesions. Also, the immunocytochemical analysis on the 10 ThinPrep specimens evidenced a strong expression of p16 in neoplastic endocervical cells. In all cases this was associated with a high-risk HPV-positive typing. p16 is a useful marker for the detection of the adenocarcinoma of the cervix uteri and its precursors. The immunocytochemical detection on ThinPrep specimens may contribute to an early detection of endocervical lesions.