Asbestosis and pleural plaques exhibit unpredictable but progressive development, and there are no markers routinely available to measure their prognosis. Asbestos exposure induces the generation of reactive oxygen species, and 8-isoprostane is involved in experimental asbestos-related lung toxicity. This oxidative stress marker was measured in exhaled breath condensate (EBC) in 92 former asbestos workers with mean age 68.8±1.7 yr and mean duration of asbestos exposure 24.1±2.0 yr. The control group had 46 subjects with mean age 65.2±3.3 yr. The mean level of 8-isoprostane, analyzed by liquid chromatography —electrospray ionization— mass spectrometry, was higher in asbestos-exposed subjects (69.5±6.6 pg/ml, p=0.0001) compared with the control group, where the concentration was 47.0±7.8 pg/ml. The results presented support the hypothesis that oxidative stress due to asbestos is the main cause of increased 8-isoprostane in EBC. Measurement of 8-isoprostane in EBC is a promising non-invasive means for assessing the activity of asbestos-induced diseases.