In a previous study, we described an elevated prevalence of human papillomavirus (HPV) in two specific types of squamous cell carcinoma of the vulva designated basaloid carcinoma (BC) and warty carcinoma (WC) compared with the conventional type of keratinizing squamous cell carcinoma (KSC). To determine whether there were other differences in their clinical presentation or behavior, we examined 100 cases of squamous cell carcinoma of the vulva classified as BC (28 cases), WC (seven cases), and KSC (65 cases). We included only cases in which tissue adjacent to the tumor was present so that the presence of intraepithelial lesions (squamous hyperplasia, lichen sclerosus, and vulvar intraepithelial neoplasia [VIN]) could be correlated with the different types of invasive carcinomas. Microscopically, BC was characterized by a relatively uniform population of small, ovoid cells with a high nuclear-cytoplasmic ratio resembling VIN 3. Although WC was similar to typical squamous cell carcinoma, it contained many squamous cells that displayed marked nuclear pleomorphism, enlargement, atypia, and multinucleation in conjunction with cytoplasmic cavitation resembling koilocytotic atypia in intraepithelial lesions. The majority of the women with BC and WC were less than 60 years of age, and the proportion of black women was higher as compared with the women with KSC, the majority of whom were white and over 65 years of age. On crude comparison, women with BC appeared to have a survival advantage compared with women with KSC; however, through multivariate modelling, when all possible confounding variables were taken into account, there was little residual impression of a survival advantage of women with BC compared with those having KSC. Substantial differences were found among the three types of carcinoma with regard to the prevalence of adjacent intraepithelial lesions. Squamous hyperplasia was found adjacent to KSC in 54 (83%) of the 65 cases, whereas 27 (77%) of 35 cases of BC and WC had adjacent basaloid or warty VIN. These findings suggest that VIN is a precursor of BC and WC. In view of the high frequency of HPV-DNA detected in VIN and in BC and WC, the findings support the view that HPV has a role in the development of these tumors. In addition, a difference was found in the distribution of associated cervical and vaginal tumors with the three types of vulvar carcinomas.(ABSTRACT TRUNCATED AT 400 WORDS)