DNA polymerase kappa (Pol kappa) is a newly identified low-fidelity polymerase implicated in spontaneous and DNA damage-induced mutagenesis. As an initial study to investigate its possible involvement in tumorigenesis, we compared the expression level of Pol kappa in tumors and adjacent nontumorous tissues by Northern blot, semiquantitative RT-PCR, and Western blot analyses. In this study, paired tumor and normal specimens from 29 patients with stages I to IIIb non-small cell lung cancer (NSCLC), including 13 adenocarcinomas, 15 squamous cell cancers, and 1 adenosquamous carcinoma, were analyzed, among which different levels of tumor-associated Pol kappa overexpression were observed in 21 of 29 matched specimens. In addition, five matched specimens exhibited elevated Pol kappa expression in both tumor and control tissues, whereas only one nontumorous tissue expressed a higher level of Pol kappa than its tumor counterpart. The preferential up-regulation of Pol kappa expression in tumors was highly significant (P < 0.001). There was no apparent correlation of Pol kappa expression levels with tumor histology, grade, and stage or with smoking history. Southern blot analysis did not show amplification of the Pol kappa gene, indicating that the elevated Pol kappa expression is likely attributable to dysregulated transcription. Our data suggest that Pol kappa may contribute to lung tumor development by accelerating the accumulation of mutations.