Necropsied lung tissues of three fatal cases with avian influenza A virus (H5N1) infection in Vietnam were analyzed to detect H5N1 virus-infected cells. Formalin-fixed and paraffin-embedded lung tissue sections showed typical histological features of diffuse alveolar damage (DAD) in all cases. Immunohistochemistry for the influenza A virus nucleoprotein antigen revealed positive signals of bronchiolar and alveolar epithelial cells in only one patient, who exhibited DAD with an exudative phase and died on the 6th day after onset. However, no signal was detected in the other two cases of DAD with a proliferative phase. These patients died on day 16 and day 17 after onset, respectively. H5N1 virus antigens were detected predominantly in epithelial cells in terminal bronchioles and in alveoli, i.e., type I and type II alveolar pneumocytes, and in alveolar macrophages. The pathogenesis of exudative DAD caused by H5N1 infection is discussed.