Human CYP2A13, which is expressed in the respiratory tract, is the most efficient enzyme for the metabolic activation of tobacco-specific nitrosamines such as 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). The relevance of CYP2A13 in carcinogenicity and toxicity in the respiratory tract has been suggested, but the expression of CYP2A13 protein in lung cancer tissues remains to be determined. We first prepared a mouse monoclonal antibody against human CYP2A13. The antibody showed no cross reactivity with the other CYP isoforms including CYP2A6. Using the specific antibody, we performed immunohistochemical analysis for human lung carcinomas. In adenocarcinomas (n = 15), all specimens were positive for the staining and five samples showed strong staining. In squamous cell carcinomas (n = 15) and large cell carcinomas (n = 15), each 14 samples were positive for the staining and two and three samples showed strong staining, respectively. In small cell carcinoma samples (n = 15), eight samples were negative for the staining and five samples showed weak or moderate staining. In conclusion, we first found that the expression of CYP2A13 was markedly increased in non-small cell lung carcinomas. The high expression might be associated with the tumor development and progression in non-small cell lung carcinomas.