Oxidative stress plays a critical role in the pathogenesis of cardiovascular disease and diabetes. Studies in vascular cells and experimental animals have demonstrated that the angiotensin type-1 receptor (AT1R) contributes to formation of reactive oxygen species by activating nicotinamide-adenine dinucleotide phosphate oxidases, but the relevance of this pathway to human heart disease has not been established. Here we demonstrate that a polymorphism in the AT1R gene (A1166C), linked to increased receptor activity, is associated with elevated levels of oxidative stress markers in heart failure patients but not in healthy controls. Plasma protein carbonyls (PCs), a marker of oxidative protein modification, were 10-fold higher in heart-failure patients compared with controls [geometric means and 95% CIs for patients, 75 (57 to 100) pmol/mg; controls, 5 (4 to 7) pmol/mg; P<0.001]. Moreover, levels of PCs were 50-fold higher in patients homozygous for the polymorphism (CC) than in controls and significantly higher than the AA and AC genotype patient groups [CC: 273 (135-550); AC: 59 (35-98); AA: 65 (40-106) pmol/mg; P<0.001]. Levels of myeloperoxidase were also modestly increased in heart-failure patients [51 (46-57) ng/mL] compared with controls [37 (32-44) ng/mL; P<0.001], but were especially elevated in patients with a CC genotype [CC: 72 (58-89); AC: 52 (44-61); AA: 39 (34-46) ng/mL; P<0.001]. The AT1R genotype was demonstrated to be an independent predictor of both PCs and myeloperoxidase levels in heart-failure patients. These findings suggest that oxidative stress in human heart failure is regulated via angiotensin signaling and may involve the nicotinamide dinucleotide oxidase pathway.