Our previous studies of lung cancer in chromate-exposed workers (chromate lung cancer) have revealed that the frequency of replication error (RER) in chromate lung cancer is very high. We examined whether the RER phenotype of chromate lung cancer is due to an abnormality of DNA mismatch repair protein. We investigated the expression of a DNA mismatch repair gene, hMLH1, and hMSH2 proteins using immunohistochemistry and microsatellite instability (MSI) in 35 chromate lung cancers and 26 nonchromate lung cancers. Lung cancer without MSI or with MSI at one locus was defined as ''RER(Γ),'' lung cancer with MSI at two loci was defined as ''RER(ΓΎ),'' and lung cancer with MSI at three or more loci was defined as ''RER(ΓΎΓΎ).'' The repression rate of hMLH1 and hMSH2 proteins in chromate lung cancer was significantly more than that of nonchromate lung cancer (hMLH1: 56% vs. 20%, P ΒΌ 0.006, hMSH2: 74% vs. 23%, P < 0.0001). In chromate lung cancer, the repression rate for hMLH1 was 43% in RER(Γ), 40% in RER(ΓΎ), and 90% in the RER(ΓΎΓΎ) group. The repression rate of hMLH1 protein in the RER(ΓΎΓΎ) group was significantly higher than that in the RER(Γ) and RER(ΓΎ) groups (P ΒΌ 0.039). The inactivation of hMLH1 expression strongly correlated with the microsatellite high instability phenotype in chromate lung cancer. The genetic instability of chromate lung cancer is due to the repression of hMLH1 protein.