Fibronectin (FN) is known to promote fibroblast proliferation and attachment of collagen fibers at the site of tissue repair or fibrosis. Although increaed levels of FN were shown in bronchoalveolar lavage (BAL) fluid of patients with interstitial pneumonia, the source of FN and the role of FN in the pathogenesis of interstitial pneumonia are still obscure. BAL fluid of patients with various types of interstitial pneumonia were obtained to measure FN and albumin (Alb) levels and to examine the cellular components. Because of the different recovery rate of BAL fluid, the FN/Alb ratio of the BAL fluid was evaluated as the real FN level in the lungs. A high FN/Alb ratio was shown in patients with idiopathic interstitial pneumonia, interstitial pneumonia with rheumatoid arthritis, hypersensitivity pneumonitis, and sarcoidosis in comparison with normal volunteers. Patients with impaired pulmonary function or acute and progressive interstitial pneumonia were shown to have a higher FN/Alb ratio than patients with normal pulmonary function or inactive interstitial penumonia. The levels of FN in BAL fuid showed a statistically significant correlation with total cell, alveolar macrophage and neutrophil concentrations of BAL fluid, but not with the lymphocyte concentration. Alveolar macrophages, isolated as adhesive cells by a brief incubation in plastic culture plates, were cultured for 48 hours, and the level of FN in the culture medium was measured. FN production by alveolar macrophages was greater in patients with interstitial pneumonia than in norman volunteers. These data suggest that the local increased production of FN might accelerate the fibrotic process in the alveolar wall, following the accumulation of alveolar macrophages and other cellular components.