ABSTRACTAmong 135 consecutive patients with myopathy, 17 showed abnormalities in the mitochondria of muscle. In eight of these patients the abnormalities were demonstrated by electron microscopy. In all patients Gömöri trichrome stain showed red masses at the sites of the mitochondria and diformazan deposits when stained for mitochondrial enzymes. The patients could be divided into three groups: i) Nine patients had weakness of the extra‐ocular muscles, either alone or associated with weakness of skeletal muscles and non‐muscular signs. One patient with the oculo‐cranio‐somatic syndrome had two affected sibs. ii) Four patients had a facioscapulohumeral distribution of weakness. Three (two sisters and a maternal aunt) were in the same family, and the fourth patient had an affected sister. iii) Four patients were clinically heterogeneous. When mitochondrial alterations were the only pathological change in many fibers in a muscle, they were most often associated with an ocular or oculo‐cranio‐somatic syndrome. Electromyography showed changes indicating or suggestive of myopathy in all but one patient. There was a discrepancy between severe changes in the EMG and nearly normal fibers by hematoxylin and eosin and van Gieson stains. Serum creatine kinase was increased in 10 of 17 patients, in most 2–4 times the upper