Oxygen radicals, and other reactive oxygen species, may play an important role in the pathophysiology of atherosclerosis, stroke, and other cardiovascular diseases. Mechanisms that account for oxidative stress in different cardiovascular diseases are diverse; for example, increases in activity of NAD(P)H oxidase, "uncoupling" of nitric oxide synthase, and maladaptive changes in expression of antioxidants can all contribute to increases in oxidative stress. Very different patterns of pro-and antioxidant mechanisms that contribute to increases in oxygen radicals in atherosclerotic plaques, hemorrhagic strokes, and aortic valve stenosis have been observed. A disappointment, in relation to the hypothesis that oxygen radicals contribute to cardiovascular risk, is that many studies indicate that antioxidant vitamins fail to reduce the risk of cardiovascular disease. Better understanding of mechanisms that lead to increases in oxidative stress in different cardiovascular diseases may lead to more effective antioxidant prevention or treatment of diseases. (Circ J 2009; 73: 201 - 207)