Background: Transcription factor FoxO1 is deacetylated in response to oxidative stress and hyperglycemia in diabetes. Results: Mice bearing constitutively deacetylated alleles of Foxo1 develop larger atherosclerotic lesions despite improved plasma lipid levels in a bone marrow transplantation-independent manner. Conclusion: FoxO1 deacetylation predisposes to atherosclerosis and vascular endothelial dysfunction. Significance: The data identify a mechanism whereby oxidative stress, acting through FoxO1 deacetylation, promotes atherosclerosis in diabetic patients.