Children less than 5 years of age with asthma were assessed for total eosinophil counts and serum levels of the eosinophil proteins, eosinophil cationic protein (ECP) and eosinophil protein X (EPX), to determine whether these measurements would reflect eosinophilic inflammation in the airways. Initially 27 symptomatic patients, 14 atopic and 13 nonatopic were investigated. They had a mean age of 1.8 years and had never been treated with inhaled steroid and had not received Intal for 2 weeks prior to the assessment. The 14 atopic patients proved to have higher mean total eosinophil counts and serum levels of ECP and EPX than the 13 non-atopic patients (eosinophil counts 0.63 x 10(9)/l vs 0.26 x 10(9)/l, P < 0.001; ECP 36.9 micrograms/l vs 10.8 micrograms/l, P < 0.001; EPX 69.0 micrograms/l vs 19.6 micrograms/l, P < 0.01). Thirteen of these patients required treatment with daily doses of inhaled steroid and 11 had a repeat assessment (seven atopic and four non-atopic). The mean serum ECP of the seven atopic patients had fallen significantly (40.6 to 22.9, P < 0.05) while the total eosinophil counts did not. These results suggest a difference in numbers and activity of eosinophils in atopic compared with non-atopic asthma in young children. To determine whether the results were influenced by treatment with inhaled steroids, 31 patients who were being treated with daily inhaled steroid underwent assessment when they were symptomatic (22 samples) or asymptomatic (19 samples). Of the 31 patients, 11 were atopic and 20 non-atopic.(ABSTRACT TRUNCATED AT 250 WORDS)