The loss of retinal pigment epithelium (RPE) with aging is related to age-related macular degeneration (AMD). This study was conducted to investigate the mechanism of hydrogen peroxide (H₂O₂) induced cell death in a human retinal pigment epithelial cell line, ARPE-19. Hydrogen peroxide was added at different concentrations to ARPE-19 cells and cultured. The cytotoxicity was assayed by mitochondrial function using 3- (4, 5-dimethylthiazol-2-yl) -2, 5-diphenyl tetrazolium bromide (MTT) testing. The patterns of cell damage were assessed using an acridine orange-ethidium bromide differential staining method, in situ end labeling (ISEL) assay and transmission electron microscopy (TEM). Catalase, a major antioxidant, was used to prevent cell death. The cleavage of procaspase 3 and poly (ADP-ribose) polymerase (PARP) was determined by western blot analysis. Hydrogen peroxide significantly induced cell death in ARPE-19 cells, whereas pretreatment of the cells with catalase prevented cell death. Application of the ISEL assay and acridine orange/ethidium bromide staining demonstrated that the H₂O₂-induced cell death occurred by an apoptotic mechanism at lower concentrations of H₂O₂ (400, 500, 600 µM), whereas higher concentrations of H₂O₂ induced necrosis rather than apoptosis. Caspase 3 was associated with the apoptotic pathway in human RPE cell death. Western blot analysis confirmed caspase 3 activation and cleavage of substrate proteins in ARPE-19 cells treated with an H₂O₂ concentration of 600 µM. These results indicate that treatment with H₂O₂ induces apoptotic and necrotic cell death in ARPE-19, and that caspase 3 is associated with apoptotic cell death. Therefore, H₂O₂ may induce the destruction of RPE cells in AMD by the combined effects of apoptosis and necrosis.