The expression of tenascin, NCAM, vimentin and HSP-70 was examined in 15 patients with dermatomyositis and 12 patients with polymyositis. All biopsies with dermatomyositis showed a confluent network of tenascin expression in the extracellular space. This network surrounded numerous fibers (39 up to 320). Tenascin expression preceded other histological and immunohistochemical signs of dermatomyositis such as the expression of N-CAM and vimentin and the appearance of perifascicular atrophy and inflammatory response. Tenascin expression was found in 3 biopsies where microtubular inclusions in the arteriolar walls were absent. In contrast, tenascin expression was confined to the surroundings of necrotic fibers also expressing vimentin or N-CAM in polymyositis. In general, tenascin was found in the vicinity of inflammatory cells, only. These findings suggest that tenascin is induced by ischemic stress in dermatomyositis. Widespread tenascin expression in a fascicular distribution is a specific marker, if an inflammatory myopathy has been proven by standard techniques. In some cases, tenascin staining might confirm the clinical diagnosis of dermatomyositis, where other histological findings are inconclusive.