Patients with unilateral acute lung injury (UALI; n = 6) and ARDS (n = 4) were evaluated by bronchoalveolar lavage, as controls we used 5 patients suffering from cerebral hemorrhage and without pulmonary, cardiac or infectious disease who were mechanically ventilated. For each group of patients two independent bronchoalveolar lavages (BAL) were performed. The BAL fluid recovered from the two lungs was immediately analyzed for leukotrienes (LTS) by means of RP-HPLC and stained for cell counts. The BAL from the control group did not show any LTS and the percentage of neutrophils was within the normal range: 1 +/- 0.2% right lung and 1.2 +/- 0.4% left lung. The BAL fluid from UALI patients showed two different patterns, the injured lung showed high levels of LTS (39.1 +/- 8 ng ml-1 LTB4; 25 +/- 6 ng ml-1 LTD4 and 27.8 +/- 8.2 ng ml-1 11-trans LTC4) and an increased percentage of neutrophils (74.2 +/- 7%) compared to controls. Only 2 out of the 6 patients from the UALI group showed small amounts of LTB4 (4 ng ml-1) and LTD4 (3.2 ng ml-1). The BAL obtained from the "healthy lung" in both cases showed values of LTS almost eight fold lower than those present in the injured lung. The percentage of neutrophils from the unaffected lungs (4.3 +/- 7%) was not significantly different from controls. Lavage fluid from ARDS patients showed a similar picture to that of the affected lung from UALI patients.(ABSTRACT TRUNCATED AT 250 WORDS)