To investigate the role of peripheral blood eosinophils and eosinophil cationic protein as parameters in monitoring oral food challenges, we monitored 25 infants and children with atopic dermatitis for up to 48 hours after 47 placebo-controlled oral food challenges with cow's milk, hen's egg, cow's milk and hen's egg, or placebo for up to 48 hours. Six healthy young nonatopic adult volunteers served as control subjects. Compared with baseline values, peripheral blood eosinophils decreased significantly immediately after clinical reaction in positive challenges (p < 0.0004), independent of the kind of reaction. Eosinophil cationic protein increased significantly 8 hours after provocation, with a maximum at 24 hours (p < 0.03). This increase was predominantly related to eczematous reactions (p < 0.005). Blood sampling immediately after clinical reaction (for eosinophils) and at 24 hours (for eosinophil cationic protein) seems to be useful in monitoring oral food challenges in children with atopic dermatitis.