Serum eosinophil (Eo) cationic protein (ECP) concentrations during acute exacerbations in asthma patients are significantly elevated compared with those during clinical remission. We measured the ratio of serum ECP concentration to peripheral blood Eo counts (ECP/Eo ratio), to determine whether the ECP release from Eo differs between the two clinical asthma situations. Forty-six children with asthma underwent spirometric assessment and blood sampling at the times of acute exacerbation and clinical remission. Twenty healthy children also were studied as a control group. The peripheral blood Eo count (468 +/- 262 per microL, mean +/- SD), the serum ECP concentration (41.7 +/- 16.9 micrograms/L), and the ECP/Eo ratio (0.104 +/- 0.049) during acute asthma exacerbations were significantly higher than the respective values during clinical remission (383 +/- 191 per microL, 27.4 +/- 11.5 micrograms/L, 0.084 +/- 0.041, all p < 0.05). The ECP/Eo ratio as well as the serum ECP concentration during acute exacerbations correlated significantly with the degree of airflow obstruction (both, p < 0.01). Both the increased peripheral blood Eo counts and the possible enhanced Eo activation may account for the elevated serum ECP concentration observed during acute exacerbations compared with that during clinical remission. Our results suggest a differential release of ECP by the Eo, depending on the disease status and asthma exacerbation severity.