PURPOSE: Glutathione S-Transferase Mu 3 (GSTM3) protects the lens from oxidative stress that contributes to age-related cataract (ARC) formation. We examined the expression and epigenetics of GSTM3 in lens epithelial cells (LECs) and lens cortex of ARC, and investigated the potential role of molecular changes in ARC pathogenesis. METHODS: This study included 120 ARCs and 40 controls. Expression of GSTM3, DNA methylation, and histone modification were assessed by quantificational real-time PCR, Western blot, bisulfite-sequencing PCR, pyrosequencing, and chromatin immunoprecipitation assay. Human lens epithelial (HLE) cell lines, SRA01/04 and HLEB3, were served as an in vitro model to observe the relationship between epigenetic status and GSTM3 expression. Potential transcription factors binding to GSTM3 promoter were detected by electrophoretic mobility shift assay. RESULTS: Expression of GSTM3 decreased in ARC lens tissues compared to that in the controls, which correlated with the hypermethylation of GSTM3 promoter. Lower level of GSTM3 was detected in HLEB3 than in SRA01/04, while HLEB3 displayed hypermethylation of GSTM3 and SRA01/04 did not. Compared to SRA01/04, HLEB3 displayed lower acetylated H3 and higher trimethylated H3K9 levels. After treatment with DNA methyltransferase inhibitor or histone deacetylase inhibitor, HLEB3 had an increased GSTM3 expression. Methylation of GSTM3 promoter abrogated the potential transcription factor binding. The GSTM3 expression declined in hydrogen peroxide-treated HLE cell lines. CONCLUSIONS: Expression of GSTM3 might be regulated by epigenetic changes in lens tissue. Hypermethylation in GSTM3 promoter and altered histone modification might have a role in the ARC formation. The results provided a potential strategy of ARC management by manipulating epigenetic changes.