Murine and human plaque macrophages are hypoxic. We hypothesized that hypoxia stimulates pro-atherogenic processes, including inflammation, and that reversal of plaque hypoxia by hyperoxic gas breathing will consequently alleviate plaque progression. To study whether hyperoxic carbogen gas (95% O2, 5% CO2) could increase arterial pO2, blood was samples from canulated femoral arteries of LDLR-/- mice (n=4 males, chow) subjected to a single 30 min exposure of carbogen (5L/min, normobaric). Carbogen significantly increased arterial pO2 from 97±3mmHg at baseline to 498±50mmHg after exposure (p<0.0001). Next, the effect of the carbogen-enhanced arterial pO2 on plaque hypoxia was studied. LDLR-/- mice with advanced, hypoxic plaques (11-wk-old males, n=5/group, 12 weeks of diet) were subjected to a single 90 minute exposure of carbogen or compressed air (21% O2), injected with pimonidazole to detect hypoxia and sacrificed directly after exposure. Single carbogen treatment of advanced plaques led to a dramatic 80% reduction of plaque hypoxia in the aortic arch (p=0.029) compared to control. Therefore, the effect of chronic carbogen exposure on atherogenesis was studied. LDLR-/- mice (20-wk-old males, n=15/group) were placed on a high cholesterol (0.25 %) diet for an initial 4 weeks, followed by 4 weeks of high fat diet and carbogen gas or compressed air (21% oxygen) 5 days/week for 90 min/day. Twenty-four hours after the last exposure, mice were sacrificed and plaque hypoxia and atherogenesis were studied in the aortic arch and root. Even 24 hours after the last exposure, chronic carbogen exposure resulted in a 42% decrease of plaque hypoxia in the aortic root (p=0.028) compared to control. Chronic carbogen exposure did not alter plaque size or macrophage content in aortic root or arch, but reduced necrotic core size by 37% in advanced plaques of the aortic root compared to control (p=0.0003). In parallel, carbogen treatment reduced plaque apoptosis (TUNEL+cells/total cell) (-50%, p=0.03) compared to compressed air, likely due to improved efferocytosis of apoptotic cells by MAC3+ macrophages (+36%, p=0.03). These plaque-stabilizing effects were independent of serum cholesterol, hematological parameters (Hb, Ht, erythrocyte counts), hematopoiesis and systemic inflammation (blood, spleen, lymph nodes, and bone marrow). Mechanistically, hypoxia reduced efferocytosis in vitro in bone marrow-derived macrophages and decreased gene expression of efferocytosis receptors (-52% MerTK, -56% CD36). Also, M2 gene expression was reduced (MRC1 -60%, IL10 -51%), whereas M1 genes were up-regulated (IL6 +250 fold, iNOS +178 fold), suggesting a hypoxia-mediated shift away from M2 pro-phagocytic macrophages. Thus, we conclude that carbogen exposure successfully enhanced plaque oxygenation and prevented necrotic core expansion, most likely by enhancing efferocytosis.