This study was carried out to determine whether serum eosinophil cationic protein (ECP) represents a sensitive marker for disease activity in atopic asthmatic patients during the pollen season. The study, in double-blind fashion, was performed between February and June 1994. Two groups of 10 seasonal asthmatic patients randomly received two different treatments. The first group was treated with inhaled beclomethasone dipropionate (BDP) 500 micrograms bid; the second received a matched placebo (P). At the beginning and every month, blood samples for determination of ECP and eosinophil count were collected and lung function (FEV1) and methacholine responsiveness (PD20) were performed. Subjects recorded daily symptoms of asthma, salbutamol consumption, and peak expiratory flow (PEF) values. In the P group, all indices, except FEV1, showed significant changes during the pollen season (P < 0.001). In the BDP group, significant changes were detected for symptom score (P < 0.01), salbutamol consumption (P < 0.01), and eosinophil number (P < 0.05). Between the two groups, significant differences for symptom score (P < 0.001), salbutamol consumption (P < 0.001), ECP levels (P < 0.05), eosinophil count (P < 0.02), PD20 methacholine (P < 0.02), and PEF values (P < 0.01) were detected. Changes in serum ECP significantly correlated with changes in other parameters (P < 0.001), except FEV1. Our results provide evidence that serum ECP is a sensitive marker for monitoring of the disease activity in seasonal asthma. Furthermore, it may offer a useful tool for estimating treatment efficacy.