Background: Sarcoidosis is a systemic disorder characterized by the accumulation of lymphocytes and monocyte/macrophage lineage cells that results in the formation of non-caseating granulomas. Thymus- and activation-regulated chemokine (TARC)/CCL17 is an important chemokine in the amplification of Th2 responses, which are achieved by recruiting CCR4-expressing CD4 + T lymphocytes. TARC concentrations are known to increase in the serum of sarcoidosis patients; however, its role in the assessment of severity and prognosis of sarcoidosis remains unknown. The objective of this study is to elucidate the role of TARC in sarcoidosis by investigating its expression in peripheral blood and at inflammatory sites. We also examined its relationship with clinical features., Methods: Serum levels of TARC, soluble interleukin 2 receptor, angiotensin-converting enzyme, and lysozyme were measured in 82 sarcoidosis patients. The Th1 and Th2 balance in circulating CD4 + T cells was evaluated by flow cytometry. The immunohistochemical staining of TARC and CCR4 was performed in order to identify the source of TARC in affected skin tissues., Results: TARC serum levels were elevated in 78% of patients and correlated with disease severity. The percentage of CCR4 + cells and the CCR4 + /CXCR3 + cell ratios were significantly higher in sarcoidosis patients than in normal subjects (P = 0.002 and P = 0.015, respectively). Moreover, TARC was expressed by monocyte/macrophage lineage cells within granulomas. The abundancy as well as distribution of TARC staining correlated with its serum levels., Conclusions: The present results suggest that elevations in TARC drive an imbalanced Th2- weighted immune reaction and might facilitate prolonged inflammatory reactions in sarcoidosis., (Copyright © 2018 Japanese Society of Allergology. Production and hosting by Elsevier B.V. All rights reserved.)