To clarify the Involvement of TNFα and IL‐1β in the Initiation of fibrotic lung diseases, their localization was examined by Immunohistochemistry. Fibrotic lung diseases observed were classified Into acute and old pulmonary fibrotic changes. The acute fibrotic changes included adult respiratory distress syndrome, acute interstitial pneumonia and idiopathic pulmonary fibrosis with acute exacerbation. Acute pulmonary fibrotic changes histopathologically corresponded to a mixture of the exudative and proliferative phases of diffuse alveolar damage. Both TNFα and IL‐1β were positive in the alveolar macrophages and proliferating type II pneumocytes In acute fibrotic changes. In contrast, positive cells for TNFα and IL‐1β were sparse in the areas of old fibrotic change and in the normal tissue. These findings suggest that TNFα and IL‐1β play an Important role In the Initiation of pulmonary fibrotic responses and in the architectural remodeling, irrespective of the etiology of fibrotic lung diseases.