Transfusion-related acute lung injury is an uncommon condition characterized by the rapid onset of respiratory distress soon after transfusion. Our understanding of its pathophysiology is based on animal models of complement (C5a) and antibodyinduced lung injury and a limited number of autopsies. These models suggest that transfusion-related acute lung injury is induced by granulocytes that aggregate in the pulmonary microvasculature after activation by transfusion-derived antibodies or biologically active lipids. The published autopsy reports provide little support for this model, as they are invariably confounded by underlying pulmonary infection, preexisting disease, and resuscitation injury. We report the case of a previously well 58-year-old man who died of transfusion-related acute lung injury within 2 hours of the onset of pulmonary distress; autopsy showed evidence of massive pulmonary edema with granulocyte assregation within the pulmonary microvasculature and extravasation into alveoli Electron microscopy revealed capillary endothelial damage with activated granulocytes in contact with the alveolar basement membrane. These findings provide direct support for the proposed model of transfusion-related acute lung injury pathogenesis.