To evaluate the clinical significance of eosinophil cationic protein (ECP), both serum and sputum levels were measured in 93 asthmatic children and 16 healthy children. Mean serum ECP levels of asthmatic children were significantly higher than those of control children. In asthmatic children, serum ECP levels were also significantly higher during attack. So, there was positive correlation between eosinophil count in peripheral blood cells and serum levels of ECP, whereas there was no correlation between eosinophil count in peripheral blood cells and sputum levels of ECP. Sputum levels of ECP within two days after onset of asthma attack were significantly higher than those in later days. However, there was no correlation between serum levels of ECP and sputum levels of ECP. Sputum ECP levels were indifferent to age. In conclusion, both serum and sputum ECP levels were elevated during asthma attack. In particular, sputum ECP levels were markedly elevated in the early stage of attack, but begun to decrease on the 3rd day and thereafter. Sputum ECP levels were suggested to represent the inflammatory process in the local airway.