The ABC transporter ABCA1 plays a key role in the first steps of the reverse cholesterol transport pathway by mediating lipid efflux from macrophages. Previously, it was demonstrated that human ABCA1 overexpression in vivo in transgenic mice results in a mild elevation of plasma HDL levels and increased efflux of cholesterol from macrophages. In this study, we determined the effect of overexpression of ABCA1 on atherosclerosis development. Human ABCA1 transgenic mice (BAC(+)) were crossed with ApoE(-/-) mice, a strain that spontaneously develop atherosclerotic lesions. BAC(+)ApoE(-/-) mice developed dramatically smaller, less-complex lesions as compared with their ApoE(-/-) counterparts. In addition, there was increased efflux of cholesterol from macrophages isolated from the BAC(+)ApoE(-/-) mice. Although the increase in plasma HDL cholesterol levels was small, HDL particles from BAC(+)ApoE(-/-) mice were significantly better acceptors of cholesterol. Lipid analysis of HDL particles from BAC(+)ApoE(-/-) mice revealed an increase in phospholipid levels, which was correlated significantly with their ability to enhance cholesterol efflux.