Nitric oxide (NO) end-products (nitrate and nitrite) are present in bronchoalveolar lavage (BAL) fluid of patients with inflammatory lung diseases. Reactive oxygen-nitrogen intermediates damage macromolecules by oxidation or nitration of critical residues in proteins. The goal of this study was to measure NO end-products (nitrate+ nitrite), in BAL fluid before and after the onset of acute respiratory distress syndrome (ARDS) and to determine if these products are associated with expression of inducible nitric oxide synthase enzyme (iNOS) in BAL cells and nitration of BAL proteins. We performed bronchoalveolar lavage (BAL) in patients at risk for ARDS (n = 19), or with ARDS (n = 41) on Days 1, 3, 7, 14, and 21 after onset, and measured total nitrite (after reducing nitrate to nitrite) and protein-associated nitrotyrosine concentration in each BAL fluid sample. Cytospin preparations of BAL cells were analyzed by immunocytochemistry for iNOS and nitrotyrosine. Nitrate+nitrite were detected in BAL fluid from patients at risk for ARDS, and for as long as 21 d after the onset of ARDS. Nitrotyrosine was detectable in all BAL fluid samples for as long as 14 d after the onset of ARDS (range, 38.8 to 278.5 pmol/mg of protein), but not in BAL of normal volunteers. Alveolar macrophages of patients with ARDS were positive for iNOS and nitrotyrosine, and remained positive for as long as 14 d after onset of ARDS. The BAL nitrate+nitrite did not predict the onset of ARDS, but the concentration was significantly higher on Days 3 and 7 of ARDS in patients who died. Thus, NO end products accumulate in the lungs before and after onset of ARDS; iNOS is expressed at high levels in AM during ARDS; and nitration of intracellular and extracellular proteins occurs in the lungs in ARDS. The data support the concept that NO-dependent pathways are important in the lungs of patients before and after the onset of ARDS.