It has been shown that cytochrome P4501B1 (CYP1B1) is over-expressed in variety of tumors. The objective of this study was to examine the expression and localization of CYP1B1 in normal lungs and tumor tissues of non-small cell lung cancers (NSCLC). CYP1B1 was detectable in the microsomes of normal lungs and lung tumors with Western immunoblotting. Utilizing immunohistochemical staining, we found that CYP1B1 was constitutively expressed in the cytoplasm of smooth muscle cells, but not in pneumocytes. Bronchiolar epithelia from three out of 19 normal lungs expressed CYP1B1. Among 89 NSCLC tumor tissues, more than 50% expressed CYP1B1. In addition, CYP1B1 expression was more common in adenocarcinomas than in squamous cell carcinomas. Recently, we reported over-expression of aryl hydrocarbon receptors (AhR) in lung adenocarcinomas. In the present study, we observed an association between CYP1B1 and AhR expression in 89 NSCLC tumor tissues. These results suggest that CYP1B1 overexpression in NSCLC is accompanied by up-regulation of AhR expression.