Verrucous carcinoma (VC) of the vulva is a rare variant of squamous cell carcinoma (SCC) of the vulva that afflicts older women and is characterized by a well-differentiated morphology with minimal nuclear atypia. The pathogenesis of VC is uncertain and a putative role for human papillomavirus (HPV) is doubtful. We analyzed 9 vulvar VCs from 7 patients diagnosed as VC of the vulva over the past 10 years at Brigham and Women's Hospital and Beth Israel Deaconess Medical Center. The patients ranged from 75 to 93 years in age (median, 83 years). One also involved the vagina and another coexisted with a keratinizing SCC. VC was associated with lichen sclerosus in 1 case; 7 others contained lichen simplex chronicus with verrucous architecture. In 7 cases, a distinctive noninvasive squamous epithelial proliferation, exhibiting a triad of marked acanthosis with variable verruciform architecture, loss of the granular cell layer with superficial epithelial cell pallor, and multilayered parakeratosis. We have designated these changes vulvar acanthosis with altered differentiation. In 5 of the 9 lesions, formalin-fixed, paraffin-embedded material was available for polymerase chain reaction analysis of HPV nucleic acids and all scored HPV negative. In conclusion, VC is a rare HPV-negative neoplasm that may be associated with other HPV-negative SCCs or its precursors, shares similar morphologic risk factors (lichen sclerosus and lichen simplex chronicus), and is frequently associated with an unusual intraepithelial lesion that can be distinguished from both classic and differentiated forms of vulvar intraepithelial neoplasia. The possibility that vulvar acanthosis with altered differentiation is a precursor to, or a risk factor for, vulvar carcinoma, merits further study.