An imbalance between oxidative stress and antioxidative capacity is thought to play an important role in the development and progression of chronic obstructive pulmonary disease (COPD). To assess the lung oxidative status in patients with COPD, we studied whether exhaled hydrogen peroxide (H2O2) is increased in breath condensate of patients with stable COPD (n = 12, mean FEV1 51% pred) and in patients with exacerbated COPD (n = 19, actual FEV1 36% pred) compared with a healthy control group (n = 10, FEV1 108% pred). Expired breath condensate during 15 min of tidal breathing was collected by cooling. The concentration of H2O2 was measured spectrophotometrically by means of horse radish peroxidase-catalyzed oxidation of tetramethylbenzidine. Concentrations of H2O2 (mean +/- SEM) were significantly elevated at 0.205 +/- 0.054 microM in patients with stable COPD compared with 0.029 +/- 0.012 microM in the control group (p < 0.05) and were further increased to 0.600 +/- 0.075 microM in patients with acutely exacerbated COPD (p < 0.001 compared with patients with stable COPD). Patients with pulmonary infiltrates on chest radiograph showed similar values compared with patients without obvious infiltrates. These findings demonstrate that patients with stable COPD exhibit increased oxidant production in the airways and that oxidant production increases further during exacerbations.