This article is available online at http://www.jlr.org Atherosclerosis, one of the prime causes of heart attack and stroke, has been well known as a lipid-driven inflammatory disease characterized by macrophage accumulation in subendothelial space. Apoptosis of macrophage-derived foam cells, especially in advanced atherosclerotic lesions where phagocytic clearance of dead and dying cells is defective, results in acellular necrotic lesions that are prone to plaque rupture and thrombosis formation, leading to the majority of acute vascular events (1). Thus, deeper understanding of the mechanisms of macrophage apoptosis may pave the way for developing novel therapeutic strategies to effectively prevent atherosclerotic plaque rupture and subsequent clinical complications (2). Oxidized LDL (ox-LDL) has been recognized as a major inducer, promoting foam cell formation and apoptosis in macrophages (3). In contrast, HDL exerts antiatherogenic functions such as promoting reverse cholesterol transport, suppressing inflammation, inhibiting LDL oxidation, and increasing endothelial nitric oxide production (4). However, accumulating evidence suggests that HDL, like LDL, is also susceptible to oxidative modification. Oxidized HDL Abstract Oxidized HDL (ox-HDL), unlike native HDL that exerts antiatherogenic effects, plays a proatherogenic role. However, the underlying mechanisms are not completely understood. This study was designed to explore the inductive effect of ox-HDL on endoplasmic reticulum (ER) stress-CCAAT-enhancer-binding protein homologous protein (CHOP)-mediated macrophage apoptosis and its upstream mechanisms. Our results showed that ox-HDL could be ingested by macrophages, causing intracellular lipid accumulation. As with tunicamycin (an ER stress inducer), ox-HDL induced macrophage apoptosis with concomitant activation of ER stress pathway, including nuclear translocation of activating transcription factor 6, phosphorylation of protein kinase-like ER kinase and eukaryotic translation initiation factor 2, and upregulation of glucose-regulated protein 78 and CHOP, which were inhibited by 4-phenylbutyric acid (PBA, an ER stress inhibitor) and CHOP gene silencing. Additionally, diphenyleneiodonium (DPI, an oxidative stress inhibitor), probucol (a reactive oxygen species scavenger), and toll-like receptor 4 (TLR4) silencing reduced ox-HDL-induced macrophage apoptosis, oxidative stress, and CHOP upregulation. Moreover, HDL isolated from patients with metabolic syndrome induced macrophage apoptosis, oxidative stress, and CHOP upregulation, which were blocked by PBA and DPI. These data indicate that ox-HDL may activate ER stress-CHOP-induced apoptotic pathway in macrophages via enhanced oxidative stress and that this pathway may be mediated by TLR4.