Pap smears of postmenopausal women are often misdiagnosed because of the difficulty in distinguishing atrophic epithelial cells groups only by morphological criteria. In this study we investigated the diagnostic application of immunocytochemical staining of p16INK4a on conventional Pap smear. A total of 137 cervical specimens were enrolled in this study, of which 77 and 60 cervical smears were taken from premenopausal and postmenopausal women, respectively. Two cervical smears were taken simultaneously in 68 women, one for conventional cytology and the other for immunostaining. Additional 69 cervical smears were taken from the archive, decolorized and then used for immunostaining. In premenopausal women 1 out of 14 (7.1%) with negative cytology, 7 out of 24 (29.2%) with low grade squamous intra-epithelial lesion (LSIL), all 35 (100%) with high grade squamous intraepithelial lesion (HSIL) and all 4 (100%) with squamous cell carcinoma (confirmed by histopathology) had positive staining to p16INK4a. In postmenopausal women p16INK4a positivity was observed in 4 out of 7 (57.1%) cases of LSIL, 12 out of 14 (85.7%) cases of HSIL and all 4 out of 5 (80%) different cases of carcinoma (1 cervical adenosquamous carcinoma and 3 cervical squamous cell carcinoma in situ confirmed by histopathology), but none of 34 smears with normal cytology. Twenty smears with normal cytology chosen for the negative control in this study were from the group of postmenopausal women and were as expected negative for p16INK4a immunostaining. In the group of postmenopausal women, 16 out of 60 (26.7%) cases the cytological diagnosis was established on the basis of pl6lNK4a immunostaining as being HSIL. From our preliminary study on a limited number of samples, we can however conclude that pl6INK4a immunostaining is a very useful tool for cytological diagnosis enabling to distinguish HSIL from normal, reactive or inflammatory changes.