Population studies suggest cardiovascular health benefits of consuming fruits and vegetables rich in polyphenolic compounds such as flavonoids. We reported previously that the flavonoid quercetin protects arteries from oxidant-induced endothelial dysfunction and attenuates atherosclerosis in apolipoprotein E gene knockout mice, with induction of heme oxygenase-1 (Hmox1) playing a critical role. The present study investigated the structural requirements of flavonoids to induce Hmox1 in human aortic endothelial cells (HAEC). We identified ortho-dihydroxyl groups and an α,β-unsaturated system attached to a catechol as the key structural requirements for Hmox1 induction. Active but not inactive flavonoids had a low oxidation potential and prevented ascorbate autoxidation, suggesting that Hmox1 inducers readily undergo oxidation and that oxidized, rather than reduced, flavonoids may be the biological inducer of Hmox1. To test this hypothesis, we synthesized stable derivatives of caffeic acid (3-(3,4-dihyroxyphenyl)-2-propenoic acid) containing either ortho-dihydroxy or ortho-dioxo groups. Compared with the dihydroxy compound, the quinone analog induced Hmox1 more potently in HAEC and also provided enhanced protection to arteries of wild type animals against oxidant-induced endothelial dysfunction. In contrast, the quinone analog failed to provide protection against oxidant-induced endothelial dysfunction in arteries of Hmox1–/– mice, establishing a key role for Hmox1 in vascular protection. These results suggest that oxidized forms of dietary polyphenols are the likely inducers of Hmox1 and may explain in part the protective cardiovascular effects of diets rich in these compounds.