Human papillomavirus (HPV) has been considered one of the important factors of malignant change. To investigate the occurrence of HPV DNA in primary squamous cell carcinoma of the lung, we tried to detect HPV DNA by polymerase chain reaction (PCR). Formalin-fixed paraffin-embedded tissue specimens from 8 cases of stage I (pT1N0M0) squamous cell carcinoma of the lung were examined for the presence of human papillomavirus DNA (type 16, 18 and 33) by polymerase chain reaction. DNA was extracted from double 5 microns sections of each specimen by deparaffinization and proteinase K digestion. Using the primers described previously by Shimada et al, target DNA sequences were selectively amplified through 40 cycles of 94 degrees C for 1 min, 55 degrees C for 2 min, and 72 degrees C for 2 min. When stained band was unclear, second PCR was conducted using product of first PCR as template of second PCR. Human papillomavirus type 18 was present in 1 case (12.5%) in contrast that type 16 and 33 were not detected in these cases.