To understand the relevance of allergy to the development of asthma in children, we examined basophil hislamine release (HR) with Df antigen, blood cosinophii counts, serum eosinophil cationic protein (ECP) levels, and bronchial responsiveness to methacholine (PC20) in three groups of children, including 36 asthmatics with high RAST titre for Df (group 1), 36 non-asthmatics with similarly high RAST titre for Df (group 2) and 21 non-asthmatics with negative RAST titre for Df (group 3). The amount of Df antigen inducing 50% HR from basophils did not vary significantly between group 1 and 2 (P>0.05), while none of the cells responded to higher concentrations of Df in group 3. The mean number of blood eosinophils and level of serum ECP were highest in group 1, and lowest in group 3, with group 2 being intermediate, and the differences were significant between all three groups (P<0.01). The mean PC20 value was the lowest in group 1, intermediate in group 2, and the highest in group 3, and the differences were significant between all three groups (P<0.01). While correlation studies showed that PC20 values of group 2 subjects signficantly correlated with their eosinophil numbers (r=–0.48, P<0.01) and ECP levels (r =–0.49, P < 0.01), such correlations were not found in group 1 subjects. These results suggest that the degree of the eosinophilic inflammation caused by the allergic reaction to mites is an important factor in determining the clinical expression of asthma in atopic subjects.