Oxidation of biomolecules may play a role in susceptibility to a number of diseases. However, there are few large-scale survey data describing oxidative damage that occurs in humans and the demographic, physical, or nutritional factors that may be associated with it. Such information is essential for the design and analysis of studies investigating the role of oxidative stress in health and disease. This paper presents data on levels of two biomarkers of lipid peroxidation, malondialdehyde and F(2)-isoprostanes, in 298 healthy adults aged 19-78 years. The study was conducted in Berkeley and Oakland, California, in 1998-1999. Sex was the strongest predictor of lipid peroxidation as measured by both biomarkers (p < 0.0001); it was stronger than smoking. C-reactive protein was positively associated with lipid peroxidation (p = 0.004), as was plasma cholesterol. Plasma ascorbic acid had a strong inverse relation (p < 0.001) with both biomarkers. Plasma beta-carotene was also associated with F(2)-isoprostanes. Other plasma antioxidants were not associated with lipid peroxidation biomarkers, once ascorbic acid was included in the multivariate model. Future surveys and epidemiologic studies should measure at least one marker of oxidative damage, as well as plasma ascorbic acid. These data would permit a better understanding of the role that oxidants and antioxidants play in the health of human populations.