We investigated the expression of thioredoxin (Trx) and thioredoxin reductase (TrxR) in 89 non-small cell lung carcinomas. Additionally, immortalized human bronchial epithelial cells (BEAS 2B) and four human lung carcinoma cells lines (A549, SK-MES-1, CALU-6, and A427) were studied by Western blot and reverse transcription-PCR for the synthesis of Trx and TrxR protein and mRNA expression in vitro. The histological samples were also studied for immunohistochemical p53 and proliferating cell nuclear antigen expression and apoptosis. In non-neoplastic lung, Trx and TrxR expression was seen in bronchial epithelial cells and alveolar macrophages, metaplastic alveolar epithelial cells, and chondrocytes of the bronchus. In non-small cell lung carcinomas, there was a widespread expression of Trx and TrxR with only three and eight cases negative, respectively. Trx and TrxR expression was located in both cytoplasmic and nuclear compartments of the cells. There was a statistical association between cytoplasmic and nuclear Trx or TrxR expression. Grade I-II tumors showed stronger cytoplasmic and nuclear Trx and TrxR immunoreactivity than grade III tumors. No association was found between p53 and proliferating cell nuclear antigen expression and Trx or TrxR immunoreactivity. However, apoptosis was inversely associated with nuclear Trx and TrxR positivity. In the cell lines studied, both non-neoplastic BEAS 2B cells and all of the carcinoma cell lines expressed Trx and TrxR proteins and mRNA. The results show that these redox-regulating proteins are highly expressed in lung carcinomas taking part in activation of transcriptional factors and regulation of apoptosis in non-small cell lung carcinoma. In high-grade tumors, Trx and TrxR expression is diminished, suggesting loss of redox regulation in tumors with low differentiation.