To assess the sensitivity and specificity of serum eosinophil cationic protein (ECP) in the diagnosis of asthma and evaluation of asthma severity, we conducted a prospective study to compare parameters of asthma severity, peripheral blood eosinophilia, and serum ECP concentrations in 88 children presenting to a university hospital outpatient clinic with suspected(n=59) or recently diagnosed asthma (n=29). Serum ECP correlated significantly (rs=0.676, P=0.0001) with peripheral eosinophil counts, but only weakly with atopic children(25±11 µg/l) than in nonatopic children (16±15µg/l) (P=0.01). Bronchial hyperresponsiveness had no significant correlation (rs=-0.21, P=0.30) with serum ECP. Lung function test results had no (peak flow) or only a weak(FEV1) correlation with serum ECP. In distinguishing between children with and without asthma or in assessing asthma severity, serum ECP is not superior to the peripheral blood eosinophil count. The diagnostic sensitivity and specificity of ECP in serum for detecting symptomatically active asthma, evaluated against the cutoff level of ECP in serum of 16 mg/l, were 54% and 71%, respectively.