DNA mismatch repair genes have been implicated in the pathogenesis and predisposition of certain malignancies through a mutator phenotype. In this study, we investigated, in 150 non-small cell lung carcinomas, the expression levels of hMLH1 and hMSH2 proteins in relation to loss of heterozygosity on chromosomes 3p and 2p, the mutational status of these genes’ promoters and the hot spot exons. We have demonstrated that 88 of 150 (58.6%) tumor specimens had reduced expression levels of the hMLH1 protein, whereas 85 of 147 (57.8%) specimens had reduced expression levels of the hMSH2 protein. Reduced expression levels of both proteins were observed in 51 of 150 (34%) specimens. In adenocarcinomas, the reduction of hMSH2 expression was more frequently observed than that of hMLH1 (P < 0.003),whereas in squamous cell carcinoma of the lung hMLH1 expression was more frequently reduced than hMSH2 (P < 0.006). Reduced expression of hMLH1 correlated with allelic imbalance on loci D3S1289 (P < 0.0002) and D2S391 (P < 0.05). It is of note that an inverse correlation was found between hMSH2 reduced expression and loss of heterozygosity at locus D3S1300 (P = 0.016). In addition, hMLH1 reduced expression was more frequently associated with heavy smokers, assessed by daily tobacco uptake(P = 0.018) and total smoking exposure(pack-years; P < 0.05). In addition, a correlation between hMLH1 reduced expression and nodal metastasis in squamous cell carcinoma of the lung was observed(P = 0.015). No mutations were identified in the promoters or exons examined in these two genes. These findings indicate that hMLH1 and hMSH2 gene inactivation is a common event in the development of non-small cell lung carcinoma and allelic loss seems to be a major genetic event involved in hMLH1 silencing. In addition, we propose that a putative negative regulator of hMSH2 gene may be located at the locus 3p14.