Background: Assessment of the adverse effects of oxidative stress related to air pollution is limited by the lack of biological markers of dose to the lungs.Objective: We evaluated the use of exhaled breath condensate (EBC) malondialdehyde as a biomarker of exposure to traffic-related pollution in children with asthma as part of a panel study in Mexico City.Methods: Standard spirometry and collection of EBC and nasal lavage were performed. Environmental monitoring sites were located within 5 km of the children's homes and schools. Data were analyzed by using generalized estimating equations.Results: A total of 480 samples of malondialdehyde were obtained from 107 patients with asthma, with a median level of 18.7 (interquartile range [IQR], 12.4-28.7) nmol. Ambient particulates less than 2.5 mu g/m(3) and ozone levels on the day of sampling were significantly associated with higher malondialdehyde levels. A 14.2-mu g/m(3) (IQR) increase in 8-hour moving average particulates less than 2.5 mu g/m(3) in size was associated with a 1.12-nmol increase in malondialdehyde and 4 15.9-ppb (IQR) increase in 8-hour moving average ozone with a 1.16-nmol increase in malondialdehyde. Malondialdehyde levels were inversely associated with forced vital capacity and FEV1 and positively associated with IL-8 levels in nasal lavage.Conclusion: Exhaled breath condensate malondialdehyde was related to both air pollution exposure and changes in lung function and inflammatory markers.