Hydrogen peroxide levels were measured in the breath condensate of 43 patients receiving mechanical ventilation. In 16 patients the mean breath condensate peroxide level was 1·68±0·35 μmol/l on the day they met diagnostic criteria for adult respiratory distress syndrome (ARDS). The peak breath condensate peroxide level in the 27 patients in whom ARDS did not develop was significantly lower (0·34±0·08 μmol/l). Plasma lysozyme, a measure of in-vivo neutrophil turnover, was significantly higher in ARDS than in non-ARDS patients (9·2±2·2 U/ml v 3·4±1·1 U/ml). These findings support the hypothesis that neutrophil activation and oxidant production are involved in the pathogenesis of ARDS.