In recent years, experimental data on the roles of viruses and cyclin B1 in G2/M arrest has accumulated. The aim of our study was to immunohistochemically examine a series of cervical squamous intraepithelial lesions for the presence of cyclin B1 and test for a possible correlation with morphologic findings or human papillomavirus (HPV) types detected by polymerase chain reaction. One hundred and three biopsies and/or conization specimens were examined, including high-grade lesions (25), low-grade lesions (52), biopsies showing nondiagnostic atypia (15), and biopsies without histopathologic alterations (11). Three patterns of immunopositivity were recognized, according to the level of epithelium exhibiting positively stained cells. Immunoreactivity for cyclin B1 above the basal/parabasal cells correlated with HPV presence. Cyclin B1-stained cells may represent "prekoilocytes," whose eventual progression to koilocytes would depend on several parameters related to the intricacies of HPV infection. In cases of nondiagnostic atypia immunoreactivity, cyclin B1 could be considered as a surrogate test.