To evaluate the use of eosinophil cationic protein (ECP) in monitoring disease activity in childhood asthma, serum ECP in 175 asthmatic children was assessed. Forty five patients with cystic fibrosis, 23 with lower respiratory tract infections (LRTI), and 87 healthy children were used as controls. Serum ECP concentrations (34.3 micrograms/l v 9.8 micrograms/l) were significantly higher in children with bronchial asthma than in healthy control subjects. In symptomatic patients with asthma serum ECP concentrations were increased compared with those from asymptomatic patients (40.2 micrograms/l v 14.4 micrograms/l), irrespective of treatment modalities (that is steroids, beta 2 agonists, or sodium cromoglycate). Moreover, atopy and infection appeared to be factors enhancing eosinophil activity in bronchial asthma as measured by serum ECP (58.4 micrograms/l v 36.8 micrograms/l and 68.8 micrograms/l v 42.2 micrograms/l, respectively). In a longitudinal trial, antiasthmatic treatment modalities (that is steroids) reduced serum ECP within four weeks (42.2 micrograms/l v 19.0 micrograms/l). In conclusion, the data indicate that (1) eosinophils also play a central part in childhood asthma; (2) serum concentrations of ECP in children with bronchial asthma are related to the disease severity and may thus be used for monitoring inflammation in childhood asthma; (3) eosinophil activity appears to be enhanced by atopy and infection; and (4) longitudinal measurements of serum ECP concentrations may be useful for optimising anti-inflammatory treatment in children with bronchial asthma.