Polymyositis (PM) and dermatomyositis (DM) are two major and distinct inflammatory myopathies. Cytokines, implicated in the immune process, have been recognized in the muscle tissue from PM and DM patients, but their functional in situ role has not been identified. We analyzed the expression of the signal transducer and activator of transcription 1 (STAT1), a molecule whose up-regulation indicates the interaction of cytokines, or growth factors, with their target receptors in muscle fibers and inflammatory infiltrates in PM and DM. An immunohistochemical analysis was performed using monoclonal antibodies to STAT1 in 57 muscle biopsies from 10 patients with DM, 10 with PM, and 37 controls. The profile of STAT1 up-regulation was also investigated in cultured muscle stimulated by interferon-gamma, epidermal growth factor, platelet-derived growth factor, and interleukin-2, using semiquantitative polymerase chain reaction and Western blot. High STAT1 expression was observed in many perifascicular atrophic muscle fibers from DM patients in 10/10 biopsies. In contrast, only a few muscle fibers undergoing necrosis were STAT1 positive in 2/10 patients with PM and in 2/37 controls. STAT1 reactivity was noted in most cells of the infiltrates in DM, PM, and controls. In vitro, STAT1 was stimulated by interferon-gamma but not by the other molecules studied. These results suggest that in DM, but not in PM, there is distinctive functional local cytokine activity able to increase STAT1 expression in muscle fibers. As interferon-gamma specifically activates STAT1 in vitro, this cytokine in conjunction with ischemia is probably involved in perifascicular muscle fiber pathology in DM.