The occurrence of human papillomavirus (HPV) DNA in primary lung carcinomas and in squamous metaplasia of the bronchus was studied using in situ hybridization techniques and commercially available biotinylated DNA probes to HPV subtypes 6/11, 16/18, and 31/33/35. The authors found HPV DNA in six of 20 cases of squamous cell carcinoma and one of six cases of large cell undifferentiated carcinoma. There were two cases each of the 6/11 serotypes and the 16/18 serotypes and three cases of the 31/33/35 serotypes. Infected cells of the squamous carcinomas uniformly showed koilocytosis. No case of adenocarcinoma, bronchioloalveolar carcinoma, or small cell carcinoma was positive (of 32 cases). Areas of squamous metaplasia in infected tumors showed similar HPV DNA expression in 15% of cases, especially in those with condylomatous atypia. In 5.8% of random bronchial biopsies of squamous metaplasia, HPV DNA was identified. The relationship of HPV infection to the development of upper and lower respiratory tract carcinomas is discussed.