Animal models have recently clarified the lung injury after allogeneic hematopoietic transplantation. These works have confirmed the role of donor T lymphocytes in immune-mediated inflammatory reactions in the lung. We report here a fatal case of a 3-year-old child who developed acute respiratory failure coinciding with the onset of hyper-acute graft versus host disease (aGVHD) after allogeneic peripheral stem cell transplantation. aGVHD was refractory to treatment and the patient died on day +28. Lung necropsy showed interstitial pneumonia and peribronchial and perivascular infiltration by mononuclear cells, with no viral inclusions. These findings are not specific but have been found by some authors in animal models with acute immune-mediated lung injury related with donor T lymphocytes. Immune-mediated lung injury, as defined by animal models, should be considered in patients with severe signs of systemic aGVHD while excluding other known etiologies of pulmonary disease. Am. J. Hematol. 69:56-58, 2002. © 2002 Wiley-Liss, Inc.