Inverted and fungiform papillomas of the sinonasal cavity share a common origin from the Schneiderian membrane, but they differ widely in their rates of recurrence and progression to carcinoma. To determine the role of human papillomavirus in the etiology of these lesions, 15 inverted papillomas, five fungiform papillomas, and two squamous cell carcinomas associated with inverted papilloma were examined for the presence of HPV by in situ hybridization (ISH) and polymerase chain reaction (PCR). ISH was carried out on formalin-fixed, paraffin-embedded material using HPV types 6/11, 16/18, and 31/33/35 DNA probes. Tissue DNA was amplified by PCR with HPV L1 consensus primers, and the product was detected by gel electrophoresis, Southern blotting, and hybridization with type specific probes (HPV types 6/11, 16, 18). Three of 15 inverted papillomas and two of five fungiform papillomas were positive for HPV 6/11 by ISH, whereas PCR detected HPV 6/11 sequences in two of 15 inverted and three of five fungiform papillomas. Biopsies from two patients who had serial resections contained HPV 6/11 in the original lesions and all recurrences. No HPV was detected in the carcinomas by ISH, whereas PCR detected HPV 16 in one carcinoma. These findings confirm the presence of HPV DNA sequences in both inverted and fungiform sinonasal papillomas as well as in an associated squamous carcinoma. This would suggest a role for HPV in the pathogenesis of Schneiderian membrane lesions. Furthermore, our data indicate that ISH and PCR are equally sensitive in detecting HPV in sinonasal papillomas.