To study the possible role of EBV in pulmonary carcinogenesis, surgically resected and paraffin-embedded pulmonary carcinoma specimens from 87 cases in which each specimen contained separable carcinoma tissue and normal lung tissue (twin pair) were collected to detect EBV infection and infective copy number by polymerase chain reaction (PCR) and dot blot hybridization. Of the 87 cases (174 samples), EBV genomic DNA sequences were positive in 52 (59.8%) carcinoma tissue samples and 39 (44.8%) normal tissue samples by PCR. There were no significant differences in positive rates between the two tissues. By dot blot hybridization, a significant difference was found between carcinoma (43.7%) and normal lung (8.0%) tissues. The results suggest that EBV infection may be common in both normal and carcinoma tissues, but carcinoma tissue contains higher EBV infective copy number than normal tissue. Infective copy number of EBV rather than simple EBV infection may play an important role in pulmonary carcinogenesis.