Transfusion‐related acute lung injury (TRALI) is clinically similar to the adult respiratory distress syndrome (ARDS) and has been linked to the transfusion of leukocyte antibodies in blood components. Animal model have implicated neutrophil (PMN)‐priming agents in ARDS; however, two agents were required. Previous studies showed the generation of PMN‐priming agents during blood storage. Thus the association of PMN‐priming agents with TRALI was examined. Ten patients with TRALI and 10 with febrile or urticarial reactions (control group) were evaluated. The presence of PMN‐priming activity was tested in the patients' pretransfusion and posttransfusion blood samples by incubating PMNs with these samples followed by activation of the respiratory burst. Plasma lipids were separated by normal‐phase high‐performance liquid chromatography (HPLC), and the priming activity was evaluated. The presence of leukocyte antibodies was determined in the blood donors and patients with TRALI. Significantly more PMN‐priming activity was present in the posttransfusion sera (11.4 +/− 1.8 nmol superoxide anion/min, mean +/− SEM; n=10) and plasma of patients with TRALI than in their pretransfusion sera (6.5 +/− 1.5: n=10) or in the pretransfusion and posttransfusion sera (5.1 +/− 1.3, n=10; and 4.5 +/− 1.4, n=10, respectively) and from the controls (p < 0.05). HPLC separation of lipids demonstrated that three active species were present in the posttransfusion plasma samples of TRALI patients. All the patients with TRALI had underlying clinical factors, such as infection, cytokine administration, recent surgery, or massive transfusion, while only 2 of 10 control patients had these clinical conditions. None of the donors had significant titers of HLA or HLA‐DR antibodies; however, 50 percent had weak positivity for granulocyte antibodies. TRALI is the result of two clinical events, the first being a predisposing clinical condition and the second being the transfusion of biologically active lipids in stored blood.