We analyze changes in eosinophilic cationic protein (ECP) serum levels after treatment with intranasal corticoids. Fifty-three healthy individuals (control group) and 21 patients diagnosed of allergic rhinitis, with or without bronchial asthma, were enrolled at a time when they had only nasal symptoms. Data were collected from skin Prick tests, forced spirometry, methacholine challenge, and complete blood workups, including IgE measurement, eosinophil counts and ECP serum levels determined by immunofluorescence. The patients received intranasal budesonide at a dose of 200 micrograms/24 h. ECP levels and eosinophil counts were determined before (baseline levels) and during treatment (on days 21 and 60). We found significant differences (p < 0.01) in baseline ECP levels of the controls (9.34 +/- 5.76) and patients (16.47 +/- 15.28). These values were significantly lower than baseline 21 and 60 days after treatment, although the changes between days 21 and 60 were not significant. Eosinophil counts did not fall significantly. We also found that eosinophil counts and ECP levels were correlated (r = 0.53) at baseline but not after treatment (r = 0.25). No patient experienced bronchial symptoms during the study. We conclude that ECP serum levels in patients with symptoms of rhinitis are significantly higher than levels in non symptomatic individuals. These levels fall significantly, possibly due to intranasal corticoid treatment, although eosinophil counts remain constant. ECP levels can therefore be used to monitor inflammatory activity in patients with allergic rhinitis.