Fas/Fas ligand (FasL) interaction can induce apoptosis, have a costimulatory role or act as a mechanism by which cytotoxic T cells produce target cell lysis. We used several commercially available antibodies to study Fas and FasL expression in polymyositis (PM), inclusion body myositis (IBM), dermatomyositis (DM) and normal controls. A strong Fas signal occurred on the sarcolemma, and to a lesser extent in the sarcoplasm of neural cell adhesion molecule (NCAM)-positive or developmental myosin heavy chain-positive regenerating muscle fibers and of injured fibers with presumed abortive regenerative activity, including some nonnecrotic invaded fibers in PM and IBM and some of the atrophic perifascicular fibers in DM. Most fibers within groups of atrophic fibers in IBM were strongly Fas-positive, and statistically more muscle fibers were Fas-positive in IBM compared to PM. A subset of the actively invading CD8+ T cells in nonnecrotic muscle fibers in PM and IBM, and scattered CD4+ cells in each inflammatory myopathy, had up-regulated Fas expression, probably reflecting costimulation. No FasL antibody consistently labeled the positive control tissue (testis) or intramuscular elements in control or inflammatory myopathy specimens. Our study identifies regenerating muscle fibers as the main site of Fas immunoreactivity in inflammatory myopathies, and Fas expression may be part of an activated or reactivated developmental program of new gene expression in regenerating or denervated muscle fibers. Our data plead against a specific role of Fas/FasL interaction in the immunopathogenesis of the inflammatory myopathies.