OBJECTIVE: Determination of the urinary concentration of eosinophil protein X (U-EPX) may objectively predict the severity and activity of asthma in children. METHODS: Concentrations of U-EPX in 80 non- atopic asthmatic children were compared with those in 25 healthy control children. The patients were studied during attacks and two weeks later. The severity of asthma attacks was determined according to a pre-existing score. U-EPX was measured by the specific radioimmunoassay technique (Pharmacia, Uppsala, Sweden). This measurement was correlated with the clinical and radiological investigations as well as with other variables such as blood oxygen saturation, peak expiratory rate and eosinophil count. RESULTS: U-EPX concentrations were significantly higher in all asthmatic children during attacks (139.6 11.7 microg/mmol of creatinine) than those in the control group (35.3 6.2 microg/mmol of creatinine) (p