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Mitochondrial alterations represent a common finding in muscle biopsy of sporadic inclusion body myositis (s-IBM) and in polymyositis with mitochondrial pathology (PM-Mito), although they have been occasionally reported in dermatomyositis (DM) and immune-mediated necrotizing myopathy (IMNM). However, while DM and IMNM usually show good treatment response, PM-Mito and s-IBM present a variable clinical course with poor response to therapy. The prevalence and significance of mitochondrial pathology in non-IBM myositis has not been deeply investigated, particularly if it could be a reliable marker of progression to IBM from PM-Mito and/or an index of treatment unresponsiveness and worse clinical outcome. We reviewed clinical and histopathological data from 22 patients, followed in 4 Italian and 1 French institutions, who presented with clinical and histological diagnosis of myositis showing COX-negative fibers at muscle biopsy. 16 patients [72,7%] were women; mean age was 65.7 years [range 39-78 years]. 4 patients had IMNM, 2 DM, 10 non-specific myositis (NSM), 6 Overlap myositis (OM). The mean number of COX-negative fibers was 3.5%. Mean age at muscle biopsy was 62,3 years. Only 5 patients [22,7 %] showed a complete recovery after treatment while the others had variable residual weakness. Treatment refractory and worst clinical outcome were observed in patients with higher percentage of COX-negative fibers. These findings suggest that skeletal muscle mitochondrial pathology might represent a marker of disease severity and predictor of worse treatment response in non-IBM myositis. [ABSTRACT FROM AUTHOR]