IL-4 is an essential mediator of IgE synthesis; it also upregulates the expression of the low affinity receptor for IgE (CD23) and its release in soluble form (sCD23). The involvement of IL-4 and sCD23 on the IgE synthesis control has been observed in experimental studies. IL-4 and sCD23 serum levels in asthmatic atopic children were determined in order to investigate a possible correlation between these factors and IgE levels. IL-4, total sCD23 and Dermatophagoides pteronyssinus specific IgE were determined in the serum of 19 asthmatic atopic children (aged 7 to 14) and in 13 age-matched controls. Since sCD23 may present an age-dependent variation, sCD23 and IL-4 serum levels were evaluated in 20" wheezy babies." IL-4 serum levels were significantly higher in patients as compared to controls, whereas there was no significant difference between them concerning sCD23 levels. sCD23 levels were, however, significantly higher among" wheezy babies" as compared to atopic and nonatopic children. There was no correlation between IL-4 and sCD23 serum levels, nor between any of these factors and IgE levels in all groups. In conclusion, the enhanced IL-4 levels suggest that atopic patients have a preferential activation of Th2 subset. CD23 expression is markedly influenced by age.