Although lipid peroxidation in the subendothelial space has been hypothesized to play a central role in atherogenesis, the role of vitamin E in preventing lipid peroxidation and lesion development remains uncertain. Here we show that in atherosclerosis-susceptible apolipoprotein E knockout mice, vitamin E deficiency caused by disruption of the alpha-tocopherol transfer protein gene (Ttpa) increased the severity of atherosclerotic lesions in the proximal aorta. The increase was associated with increased levels of isoprostanes, a marker of lipid peroxidation, in aortic tissue. These results show that vitamin E deficiency promotes atherosclerosis in a susceptible setting and support the hypothesis that lipid peroxidation contributes to lesion development. Ttpa(-/-) mice are a genetic model of vitamin E deficiency and should be valuable for studying other diseases in which oxidative stress is thought to play a role.