Idiopathic inflammatory myopathies, myositis, are characterized by infiltrates of T lymphocytes and macrophages in muscle tissue. Some patients with myositis are very resistant to immunosuppressive treatment. Thus new therapies are urgently required. An increased knowledge of the key molecular mechanisms in myositis would be important for development of new targeted therapies for these patients. IL-18 is a pleiotropic cytokine with proinflammatory and immunoregulatory effects. It has been shown to play an important pathogenic role in various autoimmune disorders (e.g. rheumatoid arthritis and Sjögrens syndrome). Whether IL-18 has a role in disease mechanism in myositis is not known.