TOPIC: Lung Pathology TYPE: Original Investigations PURPOSE: It has been reported that AEP histopathologically exhibits diffuse alveolar damage (DAD) with eosinophilia. However, acute respiratory distress syndrome (ARDS), acute interstitial pneumonia (AIP), and acute exacerbation of idiopathic pulmonary fibrosis, also exhibiting DAD histopathologically, are all life-threatening diseases. AFP is rapidly improved by steroids alone without remaining of fibrosis or sometimes even without treatment, whereas these diseases with the same histopathology show greatly different clinical courses, which remains a question to solve. METHODS: Here we investigated the histopathology of AEP in2surgical lung biopsy and 14 TBLB cases. Additionally, we determined the presence or absence of DAD by histopathology in these AEP cases. RESULTS: Characteristic histopathological findings of AEP consist of alveolar edema with infiltration of eosinophils and lymphocytes and edema of perivascular area and interlobular septa. The alveolar spaces showed fibrinous exudates. There were no hyaline membranes or massive intraluminal fibrosis. These histopathological findings of interstitial edema and fluid exudates are consist with radiological findings of lung edema and can explain the rapid and complete improvement. CONCLUSIONS: Because AEP dose not exhibit lung fibrosis histopathologically, it should not to be included in DAD which is associated with lung fibrosis. CLINICAL IMPLICATIONS: We reveal the true histopathology of AEP. DISCLOSURES: No relevant relationships by Hiroshi Mochimaru, source=Web Response