The mechanism of airway remodeling in asthmatic patients is poorly understood. Thrombin is a multifunctional protease that, in addition to its critical role in thrombotic processes, has also been described as inducing cellular and molecular events relevant to tissue remodeling. The present investigation was undertaken to evaluate the activity of thrombin in the sputum of asthmatic patients and its potential role in airway remodeling. The study population comprised 8 healthy subjects and 14 stable patients with bronchial asthma. The concentrations of thrombin, thrombin-antithrombin complex (TAT), and tissue factor were measured in the sputum of all subjects. The concentrations of thrombin (p = 0. 007), TAT (p = 0.01), and tissue factor (p = 0.02) in sputum were significantly higher in asthmatic patients than in healthy controls. The proliferative effects that sputum from asthmatic patients (p = 0. 01) and thrombin (p = 0.03) have on cultured human smooth muscle cells was inhibited significantly in the presence of recombinant hirudin, a specific thrombin inhibitor. Significant statistical correlation was observed between the degree of bronchial responsiveness and the sputum concentrations of thrombin (r = -0.8; p = 0.02) and TAT (r = -0.9; p = 0.01). The results of this study showed that increased thrombin generation occurs in the airway of patients with asthma and that it may play a role in the pathogenesis of airway remodeling. Further studies should be carried out to assess whether these findings are also observed in other airway diseases.