Pulmonary Infections are a frequent cause of morbidity and mortality In patients with the adult respiratory distress syndrome (ARDS), but the reason Is uncertain. Because neutrophlls are Important for lung defense and are found In Increased numbers In the bronchoalveolar lavege fluid of patients with ARDS, we compared the functional activities of neutrophlls obtained from lavage fluid and pUlmonary artery blood of 28 patients shortly after the onset of ARDS. The lavage fluids contained 81.3±9.9% neutrophlls, of which more than 95% were viable by vital dye exclusion, and the total protein concentrations were Increased (98.8±98.5 mg/dl). The production of superoxlde anion and hydrogen peroxide by the neutrophlls In lavege fluid was significantly Impaired compared with simultaneously tested pulmonary artery and normal neutrophlls, and the microbicidal activity of the IlMIge neutrophlls for Staphylococcus aureus was significantly Impaired.