It was reported that the main cause of intraepithelial neoplasm and squamous cell carcinoma of the uterine cervix is human papilloma virus infection, and that the expression of p16 is increased in cells infected by human papilloma virus. We performed an immunocytochemical staining for protein p16 in 17 cases of cervocovaginal smears initially diagnosed as atypical squamous cells of undetermined significance, to know whether the staining could help the differentiation of neoplastic cells from reactive atypical cells. Of 17 smears, 6 were diagnosed finally as high grade intraepithelial neoplasm or invasive squamous cell carcinoma by follow-up biopsy and smear, and 5 of the 6 were positive for p16. Three were diagnosed as koilocytosis, and one of them was weakly positive for p16. Eight were diagnosed as reactive atypical cells, and all of them were negative for p16. We thought that immunocytochemical staining of p16 in cervocovaginal smears could help the differentiation of neoplastic cells from reactive atypical cells.