Lung tissue from 9 patients dying in different stages of an acute respiratory distress syndrome due to septicemia was fixed by a postmortem transthoracic injection method for an ultrastruc-tural and morphometric analysis. At the initial stage of septic lung disease a widespread interstitial and alveolar edema was the essential finding. The capillary endothelium was relatively well preserved; in particular, no large endothelial gaps could be detected. In contrast, local destruction of the squamous epithelium, often covered by hyaline membranes, and a rapid proliferation of Type II epithelial cells were noted. In the subacute stage, cuboidal transformation of the epithelium and fibrotic alterations of the interstitium were the predominant findings. The extent of fibrotic alterations of lung architecture was not clearly related to the time course of the disease; on the other hand, the thickening of the air-blood barrier estimated by morphometry mirrored the disturbances of gas exchange and lung mechanics. The nonspecificity of the lesions observed in the subacute as well as in the acute stage of the disease provided further evidence of a uniform and characteristic reaction pattern of the lung that does not reflect the type and route (via blood or inspired air) of the damaging agents. Hence, it appears questionable whether lung biopsy specimens obtained from patients with acute respiratory distress syndrome can contribute to the diagnosis of the underlying disease.