We have previously shown that the serum TARC/CCL17 level reflects the disease activity of atopic dermatitis (AD). In this study we further examined this phenomenon in a larger number of patients and confirmed it. Furthermore, in a longitudinal study of patients with AD, serum TARC/CCL17 levels was examined and compared to other various laboratory markers such as serum LDH, serum IgE, and eosinophil number in peripheral blood. We found that serum TARC/CCL17 levels reflected the disease activity of AD most sensitively than the other laboratory markers. These results indicate that serum TARC/CCL17 levels can be used as a novel laboratory marker to reflect the disease activity of AD.