BACKGROUND: Bronchial asthma is associated with elevated serum levels of eosinophil products, such as eosinophil protein X (EPX), but the occurrence in urine of this substance in patients with asthma has not previously been studied. OBJECTIVE: This study was performed to clarify whether increased amounts of eosinophil granulocyte proteins in urine and serum reflect ongoing asthmatic inflammation and whether decreasing values reflect successful treatment. METHODS: Twelve children with a median age of 12.5 years who had mild or moderate atopic asthma were studied for 3 months. At the time of inclusion in the study, treatment with inhaled budesonide was initiated. Nine children of the same age without atopic disease served as control subjects. Levels of EPX, eosinophil cationic protein (ECP), and myeloperoxidase in serum and in urine (urinary EPX) were determined at inclusion and then after 3 months of treatment. Spirometry was performed on the same occasions. RESULTS: At the time of inclusion, urinary EPX and serum ECP were significantly higher in children with atopic asthma than in the control subjects (mean, 116.4 vs 43.0 μg/mmol creatinine [p = 0.004] and 37.0 vs 14.8 μg/L [p = 0.004]). In the asthma group urinary EPX, as well as serum ECP, decreased significantly after 3 months of treatment with budesonide (116.4 to 68.4 μg/mmol creatinine [p = 0.005] and 37.0 to 24.0 μg/L [p = 0.04]). At the same time, peak expiratory flow values increased significantly in the children with asthma (76.0% to 87.8% of predicted value [p = 0.005]) but not in the control subjects (87.0% to 90.1%). In the asthma group the levels of myeloperoxidase were similar to those in the control group, both at inclusion and after 3 months. CONCLUSION: Increased urinary EPX and serum ECP levels seem to reflect active atopic asthma, whereas decreased levels after antiinflammatory treatment probably reflect normalization of airway inflammation, and indirectly, improved lung function. (J Allergy Clin Immunol 1996;97:1179-87.)