We performed a prospective study in order: 1) to determine whether a correlation could be found between serum eosinophil cationic protein (ECP) levels and clinical and functional status in perennial asthmatics during a 5 month prospective study; and 2) to evaluate the relationship between allergic exposure and ECP levels in periodic asthmatics. Two groups of asthmatic patients were selected: a group of acutely ill perennial asthmatics and a group of periodic asthmatics. The acutely ill perennial asthmatics (n=22, mean age=39.4 yrs) were included on the basis of hospitalization for acute asthma. At the end of the hospitalization, there was a 5 month follow-up of clinical, functional and medication scores, as well as eosinophil counts and ECP levels. The periodic asthmatic group was composed of asthmatics sensitized to birch and tree pollens (n=10, mean age=33.8 yrs). The same measurement were performed on this group, before, during and after the pollen season. Under corticosteroid treatment in the acutely ill patients, there was a significant decrease in serum ECP levels between the first day of hospitalization and the day of discharge (mean: 23.2 microg x L(-1) and 9.5 microg x L(-1), respectively; p=0.006). No correlation was found between the clinical status, functional status and serum ECP levels during the 5 month follow-up. A significant increase in ECP levels was found in periodic asthmatics during the pollen season. Our results suggest that serum eosinophil cationic protein is a useful marker of allergen exposure and of acute asthma treatment. This could be of importance in the prevention and follow-up of allergic asthma; the value of serum eosinophil cationic protein measurements in the day-to-day management of adult asthmatics needs to be further clarified.