To test the hypothesis that apolipoprotein A-I (apoA-I) functions specifically to inhibit atherosclerosis independent of the level of high-density lipoprotein cholesterol (HDL-C) by promoting both reverse cholesterol transport and HDL antiinflammatory function in vivo, we established a murine atherosclerosis model of apoA-I deficiency in which the level of HDL-C is well maintained. ApoA-I^−/− mice were crossed with atherosclerosis susceptible low-density lipoprotein receptor^−/−/apobec^−/− (LA) mice to generate LA mice with apoA-I^+/+, apoA-I^+/−, and apoA-I^−/− genotypes. There were no major differences in the amounts of non–HDL-C and HDL-C in the plasma between different apoA-I genotypes. A significant inverse relationship was observed, however, between apoA-I gene dose and atherosclerosis in both female and male mice. Compared with LA-apoA-I^+/+ mice, serum from LA-apoA-I^−/− mice had a significantly reduced capacity to function as an acceptor of ABCA1- and SR-BI-mediated cellular cholesterol efflux, and also had markedly reduced lecithin cholesterol acyltransferase activity. In addition, LA-apoA-I^−/− mice had significantly reduced macrophage-derived cholesterol esterification and reverse cholesterol transport in vivo. There was significantly reduced plasma paraoxonase (PON-1) activity, impaired HDL vascular antiinflammatory function, and increased basal levels of monocyte chemotactic protein-1 in the plasma of LA-apoA-I^−/− mice compared with LA-apoA-I^+/+ mice. In LA-apoA-I^−/− mice, there was also greater induction of some, but not all, inflammatory cytokines and chemokines in response to intraperitoneal injection of lipopolysaccharide than in LA-apoA-I^+/+ mice. We conclude that apoA-I inhibits atherosclerosis by promoting both macrophage reverse cholesterol transport and HDL antiinflammatory function, and that these anti-atherogenic functions of apoA-I are largely independent of the plasma level of HDL-C in this mouse model.