Lung paraffin sections were studied in 35 patients who had died during the epidemic of A/H1N1 influenza, including 10, 16, and 9 persons diagnosed with exudative, proliferative, and fibrotic stages of acute respiratory distress syndrome (ARDS), respectively. Lung tissue factor (TF) expression was determined in the sections by immunohistochemistry. TF was found to express endothelium, neutrophils, macrophages, type II alveolocytes, and bronchial and metaplastic epithelia. In the exudative stage, the highest TF expression was observed by endothelial cells, the lowest was by macrophages and metaplastic epithelial cells. In the proliferative stage, TF expression was increased in the endothelium and macrophages and remained unchanged in other cells. In the fibrotic stage, TF expression in the macrophages remained at the same level as that in a previous phase, reduced in the endothelium, and remained unchanged in other cells.