Two distinct types of vulvar squamous cell carcinomas and their precursors, vulvar intraepithelial neoplasias (VIN), which differ in terms of clinical presentation and behavior, have been delineated. Human papillomavirus (HPV)-associated carcinomas are of basaloid or warty type, whereas tumors unrelated to HPV are usually keratinizing and differentiated. Thus, the major stratifying factor for vulvar carcinomas and VIN is their etiopathogenetic relationship with HPV. However, because of technical difficulties in confidently detecting HPV in tissues, this diagnosis is usually based on purely morphologic criteria, even though some overlap exists between these histologic types. Recently, the tumor suppressor protein p16 has been shown to be specifically overexpressed in HPV-related carcinomas and premalignant lesions of the uterine cervix, oral cavity, and anus, but the presence of p16 vulvar squamous lesions has not been examined. We have evaluated the immunohistochemical expression of p16 in a series of formalin-fixed, paraffin-embedded vulvar carcinomas and their putative precursors. p16 was strongly positive in all cases of basaloid/condylomatous VIN3 (30/30) and basaloid (7/7) and warty (3/3) carcinomas. In contrast, p16 was almost consistently negative in normal skin, squamous cell hyperplasia (0/20), lichen sclerosus (0/19), differentiated (simplex) VIN3 (0/11), verrucous carcinoma (0/2), and keratinizing squamous cell carcinoma (3/33, 9%). One of the keratinizing squamous cell carcinomas positive for p16 occurred in a 25-year-old woman and the other two were associated with small foci of basaloid VIN3 adjacent to the tumor, suggesting a probable relationship with HPV. p16 was positive in 6 of 10 of basal cell carcinomas. In conclusion, p16 immunostaining is a good discriminator between HPV-associated and HPV-unrelated vulvar carcinomas and VIN, although it cannot differentiate basaloid squamous and basal cell carcinoma.