Airway inflammation is fundamental to the persistence of asthma and chronic obstructive pulmonary disease. The hypothesis that induced sputum cell counts and mediators could help discriminate between asthma and chronic obstructive pulmonary disease was tested. Cell counts and the concentration of ECP and PMN elastase were measured in indu­ced sputum and serum of 21 healthy subjects, 17 patients with asthma and 6 with stable chronic obstructive pulmonary disease. Induced sputum eosinophils were significantly higher in case of asthma (8.2 ± 10.8%) than in healthy subjects (0.3 ± 0.6%) (p < 0.05), but chronic obstruc­tive pulmonary disease (2.4 ±1.9). Neutrophil counts were significantly higher in patients with chronic obstructive pulmonary disease (89.5%, S. D. 2.0%) than in those with asthma (65.8%, S. D. 23.7%) (p < 0.05). Induced sputum ECP were higher in asthma (148 ± 104 μg/l) and chronic obstructive pulmonary disease (113 ± 122 μg/l) than in healthy subjects (48 ± 37 μg/l) (p < 0.05). Eosinophil counts and ECP concentration in induced sputum could be used to dis­criminate between patients with asthma and healthy subjects. Neutrophil counts in induced sputum could be used to discriminate between patients with chronic obstructive pulmonary disease and asthma. However, measurement of ECP and PMN elastase concentration in indu­ced sputum and serum would not discriminate between patients with chronic obstructive pulmonary disease and asthma.