An extensive squamous cell papilloma of the nasal cavity and also filling the entire left maxillary sinus is reported, which showed cytological features identical to the known cytopathic effects of Human papillomavirus (HPV). Immunoperoxidase staining disclosed a few cells with nuclei positive for HPV antigens. The findings are discussed in the light of other evidence suggesting a possible viral etiology of these lesions, i.e. the high recurrence rate. The present observations together with the reports on the malignant transformation of nasal squamous cell papillomas emphasize the relevance of the recent concept on the possible involvement of HPV in human squamous cell carcinogenesis.