['Tetrahydrobiopterin (BH(4)) is a critical cofactor for nitric oxide (NO) synthesis by NO synthase (NOS). Recently, we demonstrated that disturbed flow produced by partial carotid ligation decreases BH(4) levels in vivo. We therefore aimed to determine whether atherosclerosis induced by disturbed flow is due to BH(4) deficiency and NOS uncoupling and whether increasing BH(4) would prevent endothelial dysfunction, plaque inflammation, and atherosclerosis.', 'We produced a region of disturbed flow in apolipoprotein E(-/-) mice using partial carotid ligation and fed these animals a high-fat diet. This caused endothelial NOS uncoupling as characterized by increased vascular superoxide production, altered vascular reactivity, and a change in endothelial NOS migration on low-temperature gel. These perturbations were accompanied by severe atherosclerosis, infiltration of T cells and macrophages, and an increase in cytokine production. Treatment with BH(4) recoupled NOS, decreased superoxide production, improved endothelium-dependent vasodilatation, and virtually eliminated atherosclerosis. BH(4) treatment also markedly reduced vascular inflammation and improved the cytokine milieu induced by disturbed flow.'] Do tetrahydrobiopterin deficiency and nitric oxide synthase uncoupling contribute to atherosclerosis induced by disturbed flow?