Immune-mediated mechanisms appear to play a primary role in the pathogenesis of polymyositis (PM) and dermatomyositis (DM). The serum of patients with active DM has high levels of circulating complement fragments C3b, C4b, and C5b-9 membranolytic attack complex (MAC) and demonstrates a very high C3 uptake in an vitro assay system. The MAC and the immune complex-specific C3bNEO fragment are deposited on the endomysial capillaries early in the disease and lead sequentially to loss of capillaries, muscle ischemia, muscle fiber necrosis, and perifascicular atrophy. In contrast, in PM the muscle fiber injury is initiated by sensitized CD8+ cytotoxic T cells that recognize heretofore unknown and probably endogenous muscle antigens in the context of major histocompatibility complex (MHC) class I expression. A restricted (oligoclonal) pattern of T-cell receptor with prominence of Va1, Vb6, and Vb15 genes is noted within the endomysial infiltrates suggesting that the T-cell response is antigen driven. In both PM and DM, intercellular adhesion molecule (ICAM)-1 and vascular cell adhesion molecule (VCAM)-1 are upregulated in the endomysial endothelial cells and function as ligands for the leukocyte integrins leukocyte function-associated antigen (LFA)-1 and very late activating antigen (VLA)-4, allowing activated lymphocytes to adhere to the endothelial cells and migrate to the muscle fibers. Among viruses, only the retroviruses human immunodeficiency virus (HIV) and human T-cell lymphotropic virus (HTLV)-1 have been convincingly shown to trigger PM, which is mediated by nonviral-specific, cytotoxic CD8+ cells. The treatment of inflammatory myopathies remains empirical. Many patients respond to steroids to some degree and for some period of time. Azathioprine, methotrexate, cyclosporine, cyclophosphamide, and plasmapheresis can be of mild to moderate benefit. High-dose intravenous immunoglobulin (IVIg) is a promising therapeutic modality for some patients resistant to therapies. In a controlled study, IVIg was effective in DM not only in improving the clinical symptoms but also in reversing the underlying immunopathology. The role of IVIg in PM and IBM is under study in control trials.