The idiopathic inflammatory myopathies (IIMs) dermatomyositis, polymyositis, and inclusion body myositis are characterized by myofiber degeneration and inflammation. The triggering factors of muscle autoaggression in these disorders are unknown, but infiltrating T cells may be activated locally and proliferate in situ. T-cell polarization involving reorientation of cytoskeleton and microtubule-organizing centers mediated by motor proteins may occur within inflammatory cells in the muscle. We therefore analyzed ubiquitous and neuronal kinesin superfamily (KIF) members KIF-5, dynein, and KIF4 in IIM muscle biopsies and in activated peripheral blood lymphocytes from healthy donors. Only KIF-4 was altered. Transcript levels were significantly higher in IIM muscle than in controls, and KIF4+ inflammatory cells were found in IIM muscles. In polymyositis and inclusion body myositis, KIF4+ cells were mainly located around individual muscle fibers, whereas in dermatomyositis, they were also near blood vessels. KIF4+ cells were not specific to any immune lineage, and some were Ki67+. In peripheral blood lymphocytes stimulated with mitogens, interleukin 2 or anti-CD3/CD28 antibodies, KIF4 expression was upregulated, and the protein was localized in the cytoplasm in association with lysosome-associated membrane protein 1+ and perforin+ lysosomal vesicles. These results imply that KIF4 is associated with activated T cells, irrespective of their functional phenotype, and that it is likely involved in cytoskeletal modifications associated with in situ T-cell activation in IIM.