Background: Genital lichen sclerosus (LS) has sporadically been reported to be associated with penile squamous cell carcinoma (SCC). Objective: The purpose of this study was to assess the risk of malignant degeneration in a series of male patients affected by genital LS. Methods: All cases of histologically proven epithelial malignancy associated with penile LS recorded in our pathology files over a 10-year period (1987-1997) were reviewed. Assessment for presence of human papillomavirus (HPV) was performed from paraffin-embedded tissues using polymerase chain reaction (PCR). Results: Five of 86 white and uncircumcised men with genital LS (mean age at diagnosis, 53 years; range, 22-83 years) showed malignant or premalignant histopathologic features: 3 had SCC, one had erythroplasia of Queyrat (unifocal SCC in situ), and one verrucous carcinoma. The average lag time from onset of LS was 17 years (range, 10-23 years). Histologically, transition from LS to frank neoplastic foci was evident in all cases of SCC. In these SCC cases, areas of epithelial dysplasia were well evident at the tumor periphery. In the remaining cases, the histologic findings were consistent with erythroplasia of Queyrat and verrucous carcinoma. PCR detected HPV 16 infection in 4 of the 5 cases; one SCC patient was negative for HPV. Conclusion: Malignant changes were associated with 5.8% of the cases of penile LS in our series. Therefore patients with genital LS are at considerable risk of the development of penile SCC, as well as other epithelial and in situ carcinomas, namely verrucous carcinoma and erythroplasia of Queyrat. HPV infection probably plays a major role because 4 of 5 patients were positive for HPV. Histologically, epithelial dysplasia may represent a precancerous stage before the development of neoplasia in atrophic nonproliferative LS lesions, as its presence at the tumor periphery in our SCC biopsy samples seemed to suggest. (J Am Acad Dermatol 1999;41:911-4.)