Fibrin deposition in the mucus plugs of asthmatic patients has long been known, and asthmatic sputum has been held to be important in the pathogenesis of bronchial obstruction. We examined the coagulation activity in the airways of asthmatic patients. Albumin as an index of plasma leakage into the bronchial lumen, thrombin antithrombin III complex (TAT), tissue factor, FDP, D-dimer and the TAT/D-dimer ratio as indices of coagulation and fibrinolytic markers were determined in expectorated or hypertonic saline-induced sputum from patients with acute and stable asthma, and with chronic bronchitis, and from normal control subjects. Patients with acute asthma, in comparison with patients with stable asthma or chronic bronchitis and normal control subjects, had significantly higher levels of albumin, TAT and TAT/D-dimer. The fibrin antigen was more positively stained immunohistochemically in sputum from acute asthmatics than in other sputa. In both patients with acute asthma and those with stable asthma, there was a significant positive correlation between albumin and TAT or albumin and TAT/D-dimer in the sputum. However, in normal control subjects, there was no correlation between these markers. These results suggest that the coagulation system in the airways of acute asthmatic patients is activated, that this favors fibrin deposition in the bronchial lumen and that coagulation pathways in the bronchial compartment and the degree of plasma exudation into the airways are dependently regulated in patients with asthma but not in normal control subjects.