Polymyositis and dermatomyositis (PM-DM) are forms of idiopathic inflammatory myositis. Interstitial lung disease (ILD) in PM-DM is recognized as a serious complication and a major cause of death in this disease. According to the results of immunophenotyping of lymphocytes in bronchoalveolar lavage fluid, cytotoxic T lymphocytes may be major pulmonary inflammatory cells of ILD in PM-DM. Glucocorticoids are considered the first-line drug treatment for PMDM patients with ILD, however they are often not sufficient to obtain improvement of ILD as a single agent. Furthermore, the addition of immunosuppressive drugs becomes necessary as steroid sparing agents to avoid the severe sideeffects often seen with high-dose steroid treatment. Cyclophosphamide, cyclosporin, and tacrolimus were reported to be effective in treatment of refractory ILD in PM-DM. Although other immunosuppressive agents; mycophenolate mofetil, intravenous immunoglobulin, and anti-TNF agents have appeared as promising agents for refractory PM-DM, the efficacy on ILD in PM-DM is still unknown. Even if treatment is initiated early in the course of the disease, some patients still develop irreversible fatal lung fibrosis under aggressive immunosuppressive therapy. The fastest way to find the most effective treatment may be to investigate the pathogenesis of the disease in detail before initiation of immunosuppressive therapy including glucocorticoids.