Bronchoalveolar lavage (BAL) provides a sample of lung surface lining fluid. Several pulmonary diseases appear to involve tissue damage caused by reactive oxygen species generated by inhaled oxidants or by phagocytic cells activated within the lung. We show that BAL from normal healthy controls contain chelatable redox active iron that is pro-oxidant towards free radical reactions. Adult patients with the acute respiratory distress syndrome (ARDS) are known to he under severe oxidative stress and to have high mortality rates. Patients who survived ARDS were found to have chelatable redox active iron in their lung fluid, whereas nonsurvivors had no such iron present. Severe lung leak in nonsurvivors was paradoxically associated with increased levels of transferrin and iron-binding antioxidant activity. We suggest that the presence of pro-oxidant iron in normal lung fluid is an important factor which makes the lung vulnerable to oxidative stress.