Eosinophils play a major role in the airway inflammation in asthma. to verify an association between airway responsiveness of asthmatics and eosinophil and/or ECP, we measured serum ECP in 23 children with atopic asthma during acute attacks and stable states, and in 10 normal healthy controls. Peak flows were monitored in 15 children. The data suggest that serum ECP levels were higher in acutely ill asthmatics than in stable state. With successful therapy for asthma, serum ECP levels were mostly decreased. However, rising ECP values after treatment may suggest unsatisfactory results with conventional therapy despite a predicatable PEFR. ECP should prove a helpful for monitoring airway responsiveness which had been evoked by inflammation and/or late phase reaction in asthma.