Experimental studies have shown that ketotifen fumarate inhibits reaginic antibody-mediated hypersensitivity reactions. In this study, the proliferative responses of peripheral blood mononuclear cells (PBMCs) to ovalbumin (OA) in children with atopic dermatitis (AD), who are sensitive to hen's egg, were significantly higher than those of healthy children. The proliferative responses of PBMCs to OA were dose-dependently inhibited by ketotifen in patients with hen's egg-sensitive AD. Moreover, the inhibition resulted from the effects of ketotifen on T cells. In contrast, the proliferative responses of PBMCs to phytohemagglutinin and tetanus toxoid were not inhibited by ketotifen. These results suggest that ketotifen inhibits food antigen-specific proliferative responses of PBMCs in patients with food-sensitive AD.