Thirty-one solitary bronchial squamous cell papillomas (SCPs) with variable degrees of dysplasia, one combined with larynx papilloma and small cell carcinoma in the contralateral lung, and 12 papillomas combined with invasive squamous cell carcinomas (SCCs) were investigated for the presence of human papilloma virus (HPV) DNA by in situ hybridization (ISH) and the polymerase chain reaction (PCR). Benign SCPs showed an association with HPV type 11 and rarely with type 6, whereas type 16 or 18, sometimes in combination with type 31/33/35, was found in SCPs associated with SCCs. In one patient HPV type 18- and 31/33/35-positive benign SCP preceded the recurrence of HPV 18-positive SCP (this time combined with carcinoma) by 2 years. Patients with SCP exhibiting HPV 16 or 18 positivity are at high risk for the development of SCC. Virus typing seems to be a better prognostic indicator than grading of dysplasia or age relationship. Virus typing by the PCR is more sensitive compared with ISH, but positive cells cannot be determined; ISH is less sensitive than the PCR but permits a definite designation of the cell types that have integrated HPV sequences into their DNA. Our data suggest that HPV typing should be performed in every bronchial SCP.