In spite of the development of various antibiotics, management of elderly patients with pneumonia remains an important problem. It is suggested that adult respiratory distress syndrome (ARDS) and disseminated intravascular coagulation (DIC) often occur in elderly patients with pneumonia. Although neutrophils are suggested to be involved in the genesis of these conditions, details remain unknown. We demonstrated that a highly cytotoxic substance, 9,10-epoxy-12-octadecenoate, is biosynthesized from linoleate by human neutrophils, thus it was named leukotoxin. Leukotoxin was detected in lung lavages from patients with ARDS. In these lung lavages, increases in albumin concentration and angiotensin converting enzyme (ACE) activity were also observed. Similar results were observed in lung lavages from rats after exposure to hyperoxia for 60 hours in an experimental model of ARDS. Intravenous administration of leukotoxin (100 mumol/kg) caused lung edema. Albumin concentration and ACE activity were increased in lung lavages of rats receiving leukotoxin. In contrast, these changes were not observed in rats administered with linoleate. Furthermore, administration of leukotoxin (100 mumol/kg) caused coagulation abnormality, i.e., increase in fibrin-fibrinogen degradation products, decrease in fibrinogen, and prolongation of activated partial thromboplastin time and prothrombin time. Administration of linoleate did not induce these changes. It is indicated that O2- was produced by respiratory burst enzyme located in neutrophil plasma membrane, and that hydroxyl radicals derived from O2- by Fenton reaction were responsible for leukotoxin synthesis. From our results, leukotoxin, a product of hydroxyl radicals and linoleate, might be responsible for the genesis of ARDS and DIC.