Asthma and chronic bronchitis are diseases:that may present similar symptoms. Because eosinophil granulocytes play an,important role in the pathogenesis of asthma, the assessment of eosinophilic inflammation may be useful in making a differential diagnosis of these two diseases. This study investigated the serum and sputum eosinophil cationic protein (ECP) levels in children with asthma and chronic bronchitis and compared them with controls. Fifty asthmatic patients being treated for mild or moderately severe asthma at a university hospital were enrolled in the study. Fifteen children with symptoms of cough and sputum production lasting more than 3 months were studied in the chronic bronchitis group and 25 healthy children were included in the control group. Asthmatic patients were divided into subgroups:according to the presence or absence of asthmatic exacerbations. Clinical evaluation and determination of ECP concentrations in serum and sputum were performed for each group. In-creased activity of eosinophils was found in patients with asthma as assessed by high serum ECP (mean 21.44 +/- 20.33 mu g/L) and sputum ECP (mean 129.65 +/- 125.01 mu g/L) levels. In patients diagnosed with chronic bronchitis, serum ECP levels were similar to those of the control group (mean serum ECP 11.04 +/- 10.23 mu g/L and 12.07 +/- 6.12 mu g/L, respectively). More importantly, sputum ECP levels of the chronic bronchitis group were much lower (mean 53.36 +/- 55.43 mu g/L) than those in patients with asthma (mean 129.65 +/- 125.01 mu g/L). The serum and sputum ECP levels of the asthmatic patients who were evaluated during an acute exacerbation were also higher than those in the chronic bronchitis group. Sputum ECP levels may be helpful in the differential diagnosis of asthma and chronic bronchitis in children.