Eosinophil granule proteins such as eosinophil cationic protein (ECP), eosinophil peroxidase (EPO) and eosinophil protein X (EPX) in serum and urine are indirect measures of eosinophil activity. The measures have been evaluated for prediction, diagnosis and monitoring of anti-inflammatory treatment modalities in children with asthma. Assessments in serum and urine are highly dependent on sampling procedures and must be performed under strictly controlled conditions using standardized sampling and laboratory procedures. The measures are influenced by circadian and seasonal variations. Measurement of the eosinophil granule proteins does not improve the predictive value of a family history of atopy. Due to insufficient sensitivity and specificity, the measures are not useful in the diagnosis of asthma in children, and the clinical use of eosinophil proteins in the individual child for assessment of asthma severity has not been sufficiently validated. Serum and urine eosinophil granule proteins, however, may be useful in extending our knowledge of suppressive effects on eosinophil activity of various doses, devices and administration regimens of inhaled glucocorticoids in children. Such evaluations may be performed in randomized, double-blind trials of well-defined age groups and they should include measures of compliance. One important aspect to look at would be the distinction between suppressive effects on eosinophil activity and clinically important anti-inflammatory effects. Considering the complexity of airway inflammation and the heterogeneity of childhood asthma, however, it may be too simplistic to look for a single measure of the inflammatory processes. In the future, perhaps, a combination of products of inflammatory cells may give more clinically relevant information with respect to prediction, diagnosis, monitoring and outcome of childhood asthma.