Background: The release of T H2 cytokines by food-specific T cells is thought to be important in the etiology of food allergy. It has been suggested that the activation state of food-specific T cells also plays a significant role, but this has not yet been studied at the single-cell level. Objective: Differences in the expression of cell-surface markers by cow's milk protein (CMP)-specific T cells between infants with and without cow's milk allergy (CMA) were evaluated at the clonal level. In addition, expression after the spontaneous development of tolerance of cow's milk in infants with CMA was analyzed. Methods: We established CMP-specific T-cell clones (TCCs) from blood of infants with CMA and atoplc dermatitis, from atopic controls with atopic dermatitis but without CMA, and from nonatopic controls. In addition, we established TCCs from infants with CMA after they had spontaneously developed tolerance to cow's milk. Expression levels of CD25, CD26, and CD30 by each TCC were analyzed by use of flow cytometry. Results: Cow's milk protein-specific T cells from infants with CMA expressed much higher levels of CD25 and CD30 than CMP-specific T cells from infants without CMA. Expression of CD26 was much lower than in normal controls. After development of tolerance for cow's milk, expression of CD25 and CD30 was decreased, whereas the expression of CD26 was increased to normal levels. Conclusion: Antigen-specific T cells from patients with food allergy display an increased expression of cell-surface markers of activation compared with cells of patients without food allergy. This suggests an intrinsically stronger food-specific T cell response in food-allergic patients, and points to the key role of food-specific T cells in the pathogenesis of food allergy.