Increased expression of viral E6 and E7 oncoproteins within the host cells results in an increase in cellular protein p16INK4a expression. That increase may serve as a marker for dysplastic and neoplastic cells of the uterine cervical epithelium. The aim of this study is to assess the p16INK4a protein expression in different stages of cytological abnormality in correlation with the proven high oncogenic risk human papillomavirus infection in order to demonstrate its value as the diagnostic marker. The study included cervical smear samples of 371 patient in whom the viral typization was done. In 171 patient, during their regular gynaecological examination, along with conventional Pap smear sampling an additional smear was taken. Two hundred cervix brush (Rovers Medical DevicesOss, the Netherlands) samples were obtained and analyzed by the LBC method and the ThinPrep2000 machine. All samples were analyzed cytologically, classified according to the Bethesda system, and immunostained with the p16INK4a-specific monoclonal antibody E6H4 (MTM Laboratories, Heidelberg, Germany). A significant difference is seen in p16 positivity between the cytological diagnosis of a high grade cervical squamous intraepithelial lesion and the group with mild dysplasia (chi2=146,48; D.F.=4; p<0,01) while most of the mild dysplastic lesions were p16 negative. In cases of p16 positive mild dysplastic lesions, that positivity is the result of viral integration into the host genome, which would imply lesions at high risk of further progression. The potential and value of p16INK4a protein not only as a prognostic, but also as a diagnostic factor is proved in this way. Reproducibility of the p16 staining technique renders it suitable for follow-up monitoring as well as for comparison of the cytological results.