We hypothesized that the serum eosinophil eationic protein (ECP) concentration to peripheral blood eosinophil count ratio (ECP/Eo ratio), reflecting active eosinophils, could better correlate with asthma severity in asthmatic patients, than each of these parameters alone. One hundred twenty children with mild to moderate persistent stable asthma were included into the study. At the first visit, previous asthma medications were withheld and patients were administered beta-2 agonists "as needed." At the second visit peripheral blood eosinophil count, serum ECP, sIL-2R, and sICAM-1 were measured, and spirometry and histamine challenge tests were performed. During the study, patients filled daily diary cards to assess symptoms score. One hundred seventeen patients completed the study. The univariate logistic regression analysis showed that asthma severity is related to PC20H, ECP, ECP/Eo ratio, sIL-2R, and sICAM-1. In general, patients with higher level of ECP, ECP/Eo ratio, sIL-2R, sICAM-1 and with lower PC20H exhibited the higher risk of moderate asthma. Multivariate regression analysis showed that only PC20H and ECP/Eo ratio were the best predictors of asthma severity; higher PC20H (1 mg/mL change) slightly decrease (OR = 0.656; 95% CI: 0.44-0.99) and higher ECP/Eo ratio (0.1 pg/cell change) increase (OR = 1.84; 95% CI: 1.02-3.34) a risk of moderate asthma. These data show that the ECP/Eo ratio is a better and more useful marker than ECP or peripheral blood eosinophil count separately in assessing asthma in children.