OBJECTIVE Vulvar intraepithelial neoplasia (VIN) is defined histopathologically by distinctive abnormalities of cellular maturation and differentiation. The purpose of this study was to investigate the functional properties of VIN related to expression of p16(INK4a) protein as well as to detection of human papillomavirus (HPV) type 16 by real-time polymerase chain reaction (RT-PCR) analysis. METHODS A total of 49 vulvar biopsy samples were examined by hematoxylin-eosin staining from benign/reactive lesions, condyloma acuminatum, VIN, and invasive squamous cell carcinoma (SCC). JC8 mouse monoclonal antibodies were used that recognize p16(INK4a) epitope at a dilution of 1:25. The reaction pattern for p16(INK4a) was graded in each sample between 0 and 3+. RT-PCR analysis of formalin-fixed paraffin-embedded sections determined positivity for HPV type 16. RESULTS p16(INK4a) immunoreactivity was different in VIN 1, VIN 2, VIN 3, and squamous cell carcinoma. Strong expression of p16(INK4a) protein was observed in 92% (22 of 24) of VIN 2 and VIN 3 lesions and 100% (4 of 4) of invasive SCCs. Two (67%) of 3 VIN 2 lesions, 17 (81%) of 21 VIN 3 lesions, and 4 (100%) of 4 SCCs were positive for HPV type 16 by PCR analysis. Two (20%) of 10 VIN 1 lesions were immunoreactive for p16(INK4a), with only 1 lesion positive for HPV type 16. No p16(INK4a) immunoreactivity was observed in any of the benign/reactive and condyloma acuminatum lesions. In addition, none of the benign/reactive or condyloma lesions were positive for HPV type 16 by RT-PCR analysis. CONCLUSIONS Upregulation of INK4a gene occurs in vulvar carcinogenesis. p16(INK4a) is not a sensitive marker for differentiation of benign vulvar squamous epithelium from condyloma acuminatum or VIN 1 lesions because most VIN 1 lesions are p16(INK4a) negative. Expression of p16(INK4a) may aid in the diagnosis of HPV-related lesions and as such may be of value as a surrogate marker in the diagnosis of vulvar premalignant and malignant lesions.