Objective : The aim of this study was to investigate the usefulness of serum ECP as a marker of the severity of asthma and extent of airway inflammation. Methods : We investigated 108 patients suffering from bronchial asthma, who were classified as mild intermittent(n=19), mild persistent(n=27), moderate persistent(n=42), and severe persistent(n=20) and 10 healthy controls. Atopy was defined as those who showed >2+ responses on skin prick test. Serum ECP, peripheral blood eosinophil, sputum eosinophil, and PEFR were measured on the same date and methacholine PC20 were determined within 2 weeks. Results : Serum ECP levels were 10.1±2.0ug/L in controls, and 29.1±23.6ug/L in asthmatic patients. According to symptom severity, serum ECP levels were 22.9± 15.6ug/L, 28.6±24.1ug/L, 29.5± 22.2ug/L, and 34.6±31.2ug/L in mild intermittent, mild persistent, moderate persistent and severe persistent asthmatic patients, respectively and there were no significant differences among four groups(p>0.05), Serum ECP levels correlated with peripheral blood eosinophil counts(r=0.48, p<0.01), but not with sputum eosinophil, PEFR, and methacholine PC20 levels. There was no significant difference in serum ECP level between atopic and non-atopic asthma(p>0.05). Conclusion : Single measurement of ECP level at clinic could not represent the severity of asthma.