Oxidative stress is elevated in obesity, and may be a major mechanism for obesity-related diseases. Nonsmokers (n=396) were randomized to 1000 mg/day vitamin C, 800 IU/day vitamin E, or placebo, for 2 months. Treatment effect was examined in multiple regression analyses using an intention-to-treat approach. Vitamin C (P=0.001) and vitamin E (P=0.043) reduced plasma F2-isoprostanes. In the overall sample, changes from baseline were +6.8, -10.6, and -3.9% for placebo, vitamin C, and vitamin E groups, respectively. However, a significant interaction with baseline F2-isoprostane was found. When baseline F2-isoprostane was >50 microg/mL, vitamin C reduced F2-isoprostane by 22% (P=0.01). Vitamin E reduced it by 9.8% (P=0.46). Below that cut point, neither treatment produced further reductions. F2-isoprostane>50 microg/mL was strongly associated with obesity, and was present in 42% of the sample. Change in malondialdehyde concentration was minimal. These findings suggest a role for vitamin C in reducing lipid peroxidation. Future research on effects of vitamins C or E on plasma F2-isoprostane should limit participants to those with baseline levels >50 mug/mL. Further studies are needed to establish whether treatment with vitamins C or E in persons with concentrations above that cut point could slow the development of cardiovascular disease.