We evaluated 10 patients with histologically typical polymyositis except for an excess of muscle fibres with absent cytochrome oxidase (COX) staining. No biopsies had vacuoles or congophilic material in muscle fibres. All patients presented with a history of slowly progressive weakness. The average age of onset was 9 years older than a group of polymyositis patients with normal COX staining of muscle fibres. Selective weakness of knee extension was a prominent and disabling feature in most patients. Serum creatine kinase was usually mildly elevated (363 +/- 115 U/l) but at levels lower than those in other patients with polymyositis. Mitochondrial DNA analysis showed multiple deletions in 90% of muscles from patients with excessive numbers of COX-negative muscle fibres, a prevalence significantly greater than the 22% figure for controls (P = 0.005). As a group, the patients responded poorly to immunosuppressive therapy. We conclude that patients with polymyositis and an excess of COX-negative muscle fibres, but no inclusion bodies, have common features including selective quadriceps weakness, mitochondrial pathology by histochemical and DNA analysis and a poor response to immunosuppressive therapy. Some of these features are shared with inclusion body myositis (IBM) and this entity cannot be entirely excluded as vacuoles may not be present in all muscle tissue in IBM patients. Evaluation of the COX activity in muscle fibres of patients with inflammatory myopathies provides useful prognostic information regarding the likelihood of improved strength after immunosuppressive treatment.