Cytokines have been shown to be potent inducers of major histocompatibility complexes (MHC) class I and II as well as of cell adhesion molecules in muscle tissue cultures, indicating that cytokines may play a role in mediating muscle fiber damage in inflammatory myopathies. We found in 21 cases of autoimmune myositis various amounts of inflammatory cells expressing interleukin (IL)-1α and -β, IL-2, IL-4, tumor necrosis factor (TNF) -α and -β, and interferon (IFN)-γ and its receptor. Muscle fibers displayed enhanced expression of IL-1α and -β, IL-2, and TNF-α. Upregulation of cytokines was strongest at sites of cellular infiltration typical for the respective myositis subtype. There was no correlation between the cytokine expression and the grade of inflammation. To a lesser extent, cytokines were also present in Duchenne muscular dystrophy expressed by muscle fibers positive for TNF-α and by phagocytic mononuclear cells. Expression of cytokines by the muscle fiber may enable the muscle fiber to induce and mediate the process of autoimmunization and antigen-expression by itself without primary presence of inflammatory cells. Cytokine-expressing muscle fibers may enhance the cytolytic potential of cytotoxic cells and the muscle fiber may serve as source and target.