Infection with respiratory virus has been shown to exacerbate asthma in humans. However, the role of a respiratory virus in the pathogenesis of chronic asthma and/or wheezing in young children has not been clearly defined. It has also been debated whether virus-induced wheezing in young children is one entity and allergic asthma another, or whether they are different expressions of the same disease. The present study was done to compare ECP concentrations in nasopharyngeal secretions and serum from 32 nonasthmatic wheezing children with viral infections (RSV in 15 children; influenza B virus in 17 children detected by immunofluorescence antibody technique), 8 asthmatic children without viral infections, and 13 normal children as the controls to understand the role of eosinophil inflammation. The geometric mean of ECP in nasopharyngeal secretions was significantly higher in asthmatic children than in children with virus-induced wheezing (p < 0.05). ECP levels of nasopharyngeal secretions from children with the virus-induced wheezing were significantly greater than those of the controls. However, there were no significant differences in ECP levels in serum among subjects.