PM, IBM, and DM represent the commonly occurring inflammatory myopathies. In DM, the effector response appears to be predominantly humoral and directed against intramuscular blood vessels; a local humoral response may occur in muscle itself. Capillary lysis precedes other pathologic changes, suggesting that the capillary endothelium is an early and possibly primary target of the immune response. Questions remain about the role of immune complexes versus circulating antibodies, whether the muscle fiber injury can be explained by ischemia alone, and the possible role of endomysial CD8+ T cells. In PM and IBM, there is evidence for T-cell mediated cytotoxicity against the muscle fibers; however, muscle fiber destruction also could result from antibody-dependent complement-mediated lysis of the sarcolemma. Questions remain about the identity of the muscle fiber surface antigen(s) recognized by T cells and the molecular mechanisms of T-cell-mediated muscle fiber destruction.