An oxidant/antioxidant imbalance has been proposed in patients with idiopathic pulmonary fibrosis (IPF). We tested this hypothesis by measuring various parameters of the oxidant/antioxidant balance in the plasma of 12 patients with IPF (7 nonsmokers and 5 smokers); in the bronchoalveolar lavage fluid (BALF) of 24 patients with IPF (17 nonsmokers and 7 smokers) and 31 healthy subjects (23 nonsmokers and 8 smokers). The trolox equivalent antioxidant capacity (TEAC) in plasma and BALF was lower in nonsmoking patients with IPF (plasma 0.55+/-0.1 mM, p<.001; BALF 4.8+/-1.2 microM, mean +/-SEM, p<.01), compared with healthy nonsmokers (plasma 1.33+/-0.03 mM; BALF 10+/-2 microM). Similar trends in plasma and BALF TEAC were observed in smoking patients with IPF in comparison with healthy smokers. The decrease in BALF TEAC was concomitant with a decrease in BALF protein thiol levels, but the decrease TEAC levels in plasma in IPF patients was not accompanied by a decrease in protein thiol levels. Reduced glutathione (GSH) was lower in BALF in nonsmoking patients with IPF (1.0+/-0.1 microM) compared with healthy nonsmokers (2.3+/-0.2 microM, p<.001). In contrast, GSH levels were higher in smoking patients with IPF (5.2+/-1.1 microM, p<.001) than in nonsmoking patients. GSSG levels were not different in any of the groups. The levels of products of lipid peroxidation measured as thiobarbituric acid reactive substances (TBARS) in plasma and BALF were significantly increased in both smoking (plasma 2.2+/-0.5 microM, p<.01; BALF 0.18+/-0.04 microM, p<.001), and nonsmoking (plasma 2.1+/-0.3 microM, p<.01; BALF 0.22+/-0.05 microM, p<.001) IPF patients, compared with healthy nonsmokers (plasma 1.4+/-0.3 microM; BALF 0.05+/-0.004 microM). These data show evidence of oxidant/antioxidant imbalance in the lungs of patients with IPF, which is also reflected as systemic oxidant stress.