Molecular pathologic study of human papillomavirus infection in inverted papilloma and squamous cell carcinoma of the nasal cavities and paranasal sinuses. Nasal inverted papilloma is a rare benign tumor occasionally associated with squamous cell carcinoma. To determine the etiological role of human papillomavirus in inverted papilloma, and to clarify the relationship between human papillomavirus and malignant transformation of this benign tumor, we retrospectively analyzed inverted papillomas from 26 patients, 7 of whom had squamous cell carcinoma. We used an immunohistochemical method and molecular pathologic techniques, or dot-blot hybridization of DNA extracted from paraffin-embedded tissues, in situ hybridization, and polymerase chain reaction. Human papillomavirus was detected in 5 of 26 patients (19%), 3 patients with human papillomavirus 11 and 2 patients with human papillomavirus 16. The latter 2 patients had inverted papillomas associated with squamous cell carcinoma. We speculate that human papillomavirus may be related to the malignant transformation of inverted papillomas. 