Fibronectin is normally present in the lower respiratory tract. Significantly increased levels of it were detected in the lavage fluid in patients with interstitial lung diseases. Because this molecule appears to mediate a number of components of the inflammatory process, we evaluated the status of fibronectin in plasma and bronchoalveolar lavage in patients with recurrent obstructive bronchitis when signs of severe chronic mucosal inflammation were observed bronchoscopically. There was no considerable difference in plasma concentrations of fibronectin obtained from healthy children and patients. A comparison of lavage fibronectin/albumin ratios with plasma fibronectin/albumin ratios suggested significant local production, especially when the lavage and plasma ratios were measured in the same patients. Phagocytic activity of alveolar macrophages and blood granulocytes from the same patients was enhanced at both concentrations of fibronectin used. This concentrations referred to values quantified in the lavage fluid. The metabolism of fibronectin seems to be an important factor in tracing the inflammation process not only in adults with chronic interstitial lung diseases, but also in children with recurrent obstructive bronchitis.