Background: Atopic dermatitis (AD) is a chronic and relapsing inflammatory skin disease characterized by the predominant infiltration of T_H2-type cells in lesional skin. Thymus and activation-regulated chemokine (TARC/CCL17) is a chemokine that attracts CC chemokine receptor 4–positive (CCR4⁺) or CCR8⁺ cells. Objective: The purpose of this study was to investigate the participation of TARC in AD. Methods: We measured serum TARC levels in 40 patients with AD, 20 healthy control subjects, and 20 patients with psoriasis. We also examined disease activity by using SCORAD score; serum soluble E-selectin, soluble IL-2 receptor, IgE, and GM-CSF levels; and eosinophil numbers in peripheral blood, as well as correlations between TARC levels and these factors. The positivity of CCR4 of CD4⁺CD45RO⁺ cells in PBMCs was examined by using FACS analysis. Immunohistochemical staining of TARC and GM-CSF was performed in the lesional skin of patients with AD. Results: The serum TARC levels of patients with AD were significantly higher than those of healthy control subjects and patients with psoriasis. The serum TARC levels significantly correlated with eosinophil number (r = 0.61), SCORAD score (r = 0.60), and serum soluble E-selectin levels (r = 0.58) and weakly correlated with serum soluble IL-2 receptor levels (r = 0.34) in patients with AD. The TARC levels of patients with AD decreased after the treatment in accordance with the improvement of clinical symptoms. The CCR4 positivity of CD4⁺CD45RO⁺ cells in PBMCs of patients with AD was also higher than that of healthy control subjects. Immunohistochemical staining revealed that TARC was positive in ke-ratinocytes in the epidermis and in vascular endothelial cells, T cells, and dendritic cells in the dermis. Conclusion: Serum TARC levels are associated with disease activity of AD, and TARC may play an important role in the pathogenesis of AD. (J Allergy Clin Immunol 2001;107:535-41.)