The number and significance of airway eosinophils in stable COPD is controversial. Aims of this study were to evaluate airway inflammation in patients with stable COPD compared with other groups, and to examine the correlations between inflammatory markers and functional indices of airway obstruction. Cellular analysis and evaluation of eosinophil cationic protein (ECP) levels in induced sputum were made in 46 subjects (10 patients with clinically stable COPD, 15 patients with asthma, 11 asymptomatic smokers, and 10 healthy control subjects). As expected, eosinophils were significantly (p < 0.01) higher in patients with asthma (22.2%) than in other groups (COPD, 0.7%; smokers, 0.2%; control subjects, 0.2%), and neutrophils were significantly (p < 0.01) higher in patients with COPD (77.5%) than in the other groups (asthma, 26.7%; smokers, 33.1%; control subjects, 35.9%). However, eosinophils were also increased in patients with COPD, as compared with healthy controls (p < 0.05). Sputum ECP levels were significantly and similarly higher in both asthma and COPD groups than in the other two groups (p < 0.01). In patients with COPD and asymptomatic smokers, considered as a whole, good correlations were found between eosinophils and ECP, on the one hand, and between FEV(1) and the FEV(1)/FVC ratio, on the other. Our data suggest that eosinophils may be involved in the airway inflammation of COPD.