Eosinophil cationic protein (ECP) is a cationic protein secreted by eosinophils with toxic properties for the respiratory epithelium. Sputum-ECP levels have been shown to correlate inversely with airflow obstruction in asthma. In the present study we investigated whether ECP concentrations are different between asthmatic patients and patients with chronic bronchitis. Sputum-ECP concentrations from seven patients with bronchial asthma and seven patients with chronic bronchitis matched for FEV1 were compared (FEV1 Asthma: 66.1 +/- 29.0% of predicted; FEV1 Chronic Bronchitis: 65.2 +/- 33.3% of predicted; p = n.s.). Furthermore, sputum-ECP levels in 4 asthmatic patients with severe airflow obstruction and in 1 patient with chronic bronchitis were measured before and after initiation of a 7-day oral therapy with methylprednisolone 20 mg BID. Changes in sputum-ECP values were compared with changes in FEV1 in these 5 patients. Sputum-ECP levels and pulmonary function were measured as previously described (Am Rev Respir Dis 1992: 145: 604). Sputum-ECP levels from asthmatics were significantly elevated compared with patients with chronic bronchitis: asthma: 893.4 +/- 346.2 micrograms/l per g sputum; chronic bronchitis: 30.0 +/- 8.5 micrograms/l per g sputum (p = 0.002). The degree of airway obstruction correlated with the sputum-ECP levels in asthmatic (r = 0.76, p = 0.05), but not in the patients with chronic bronchitis (r = 0.24, p = n.s.).(ABSTRACT TRUNCATED AT 250 WORDS)