The renin-angiotensin system (RAS) plays an essential role in fluid and electrolyte homeostasis and the regulation of vascular tone; however, dysregulation and over-activation of the RAS lead to the pathogenesis of various cardiovascular diseases. The RAS is closely associated with NADPH oxidase, a major enzymatic source of reactive oxygen species (ROS) in vasculature, and angiotensin II, the final effecter of the RAS, is a potent stimulator of this oxidase. There are accumulating evidences to support the significance of NADPH oxidase in the pathogenesis of atherosclerosis. We demonstrated that the expression of NADPH oxidase is markedly enhanced in human atherosclerotic coronary arteries, and the distribution of oxidized oxidized low-density lipoprotein (LDL) in vasculature is closely associated with NAPDH oxidase and ROS. Our series of observations indicate there is a vicious circle consisting of vascular NADPH oxidase, the RAS, ROS, and oxidized LDL. Furthermore, we demonstrated that angiotensin II type 1 receptor blockers (ARBs) significantly suppressed the expression of NADPH oxidase p22phox in the aortic walls of patients with thoracic aortic aneurysm. ARBs, widely used for treatment of hypertension and hypertension-related organ damage, have succeeded in reducing the onset of cardiovascular diseases, preventing organ damage, and cardiac death. These beneficial effects of ARBs are largely dependent upon their primary effects of blood pressure lowering. However, this group of agents exerts a wide variety of biological effects on vascular metabolism, including antioxidative and anti-inflammatory actions. These pleiotropic actions play a role in cardiovascular protection. From a viewpoint of oxidative stress, we discuss pleiotropic effects of ARBs on vascular metabolism focusing on pathogenesis of atherosclerosis-based cardiovascular diseases.