OBJECTIVE: Idiopathic inflammatory myopathies (IIMs) are systemic autoimmune diseases characterized by infiltration of T lymphocytes, monocytes, and macrophages in muscle tissues. Interleukin-17 (IL-17), a Th17 cytokine, has potent pro-inflammatory actions and plays a role in autoimmune diseases. We investigated the expression of IL-17 in muscle tissues of patients with IIMs. METHODS: We measured the IL-17 mRNA level of muscle tissues from 14 patients with IIMs (9 patients with dermatomyositis and 5 patients with polymyositis) by real-time RT-PCR and compared with controls. We also performed an immunohistochemical stain to detect IL-17 expression. RESULTS: The expressions of IL-17 were significantly enhanced in IIMs than controls. In immunohistochemistry, IL-17 was expressed in perimysial, endomysial and perivascular infiltrating inflammatory cells. CONCLUSION: These results suggest that IL-17 plays a role in the immunopathogenesis of IIMs.