Background: In epidemiologic studies, it may be difficult to identify children with bronchial asthma. Since this is the most common chronic respiratory disease in childhood, and its prevalence is still increasing, reliable methods for identification of asthmatic children are required. This study evaluates the use of urinary eosinophil protein X (U-EPX) in epidemiologic studies in identifying atopic and asthmatic children. Methods: U-EPX was measured in 877 Austrian schoolchildren. The skin prick test (SPT) was performed with eight common aeroallergens, and established questionnaires were used to assess respiratory symptoms. Results: Of our cohort, 2.8% reported physician-diagnosed asthma, 5.1% reported wheezing within the last 12 months, and 24.1% were found to be atopic. In children with physician- diagnosed asthma, as well as in atopic children (positive SPT), median U-EPX levels were significantly higher than in healthy subjects (142.8 and 89.6 vs 63.9 μg/mmol creatinine, P < 0.0001 and P < 0.0001, respectively). In addition, perennial sensitization to inhalant allergens resulted in higher U- EPX levels than did seasonal sensitization. The odds ratio for U-EPX levels over the 90th percentile was significantly elevated for asthma, for wheezing, for nocturnal cough, and for breathlessness at exercise, as well as for seasonal and perennial sensitization. Pulmonary function was negatively related to U-EPX levels. Conclusions: Measurement of U-EPX, which can be obtained easily, may be helpful in diagnosing both asthma and atopy in children. However, there is a great overlap between controls and symptomatics, a fact which reduces the sensitivity of U-EPX in determination of the prevalence of asthma in epidemiologic studies.