Interleukin (IL)-10 is a potent regulatory cytokine that decreases inflammatory responses. This study investigated whether IL-10 levels in the airway are decreased in chronic airway inflammation associated with asthma or chronic obstructive pulmonary disease (COPD). Sputum was obtained from 12 healthy nonsmokers, 10 healthy smokers, 16 asthmatic patients and seven patients with COPD by means of the sputum-induction method. The IL-10 level was measured via enzyme-linked immunosorbent assay and immunocytochemical analysis. The IL-10 level in sputum was significantly lower in asthma and COPD patients and healthy smokers compared with that in healthy nonsmokers (nonsmokers, 68.0+/-11.3; smokers, 45.3+/-7.8; asthma, 26.7+/-4.0; COPD, 18.0+/-2.3 pg x mL(-1); p<0.05 for nonsmokers versus the other groups). The percentage of IL-10-positive cells in the sputum was also significantly lower in asthma and COPD and in smokers (nonsmokers, 13.2+/-1.7; smokers, 6.4+/-1.8; asthma, 5.4+/-3.5; COPD, 3.5+/-1.6%; p<0.05 for nonsmokers versus the other groups). The IL-10-positive cell appeared morphologically to be the macrophage. These data suggest that the reduced level of interleukin-10 within the airways plays a role in the pathogenesis of chronic airway inflammation in asthma and chronic obstructive pulmonary disease.