AIM: To investigate whether there is loss of the p27Kip1 protein in developing cervical cancer and whether p27Kip1 immunoreactivity has any relation to the proliferative indicator Ki-67. METHODS: The expression of p27Kip1 and Ki-67 was assessed by immunohistochemistry in serial sections from normal epithelium (13), low grade (27) and high grade (19) squamous intraepithelial lesions (LSIL, HSIL), and invasive cervical cancer (23). In the SIL cases the presence of human papillomavirus (HPV) genomic sequences was assessed by in situ hybridisation. The results were evaluated by image analysis, and reported as mean score of the percentage of p27Kip1 and of Ki-67 positive cells in each histological group. RESULTS: In general, p27Kip1 immunostaining was related to squamous differentation, and was intense in normal epithelium (47%), while it was reduced in SIL lesions as an effect of the decreased number of differentiating cells. However, decrease in the p27Kip1 expression was more evident in LSIL (36%) than in HSIL (39%); in the latter, p27Kip1 had a different intraepithelial distribution in that the staining extended to the basal cells. The average levels of p27Kip1 were similar in SIL lesions associated to low, intermediate, and high risk HPV types. Compared with normal epithelium and dysplasia, invasive cancer showed significantly lower p27Kip1 levels (23%). There was no relation between p27Kip1 and Ki-67 labelling indices in any of the histological groups examined. CONCLUSIONS: A reduction in p27Kip1 protein occurs in cervical cancer independently of the proliferative status. The changes in p27Kip1 expression may be related to the unregulated kinetics of developing cervical cancer.