The aim of the present study was to investigate whether sputum eosinophil cationic protein (ECP) concentrations could be a useful marker in the differential diagnosis between intrinsic asthma and chronic obstructive pulmonary disease (COPD). For this purpose total blood eosinophil counts were obtained and concentrations of serum and sputum ECP from 10 nonatopic asthmatics with a mild attack and 9 COPD patients with acute exacerbation were measured by radioimmunoassay. Mean serum ECP concentration was 54.3 +/- 23.0 micrograms/L in the asthmatic group and 83.3 +/- 79.2 micrograms/L in the COPD group (p: n.s.). In the group of asthmatics mean sputum ECP level was 984.5 +/- 1245.5 micrograms/L/g sputum and in the COPD group it was 417.5 +/- 363.5 micrograms/L/g sputum. There was no significant difference in sputum ECP levels between patients with asthma and COPD. We conclude that neither sputum nor serum ECP levels are useful markers in differential diagnosis of asthma attack and acute exacerbation of COPD.