Nerve conduction studies, conventional and quantitative concentric needle EMG and single fibre ENG were performed on 5 clinically affected and 7 clinically asymptomatic members of a family with a mitochondrial myopathy manifesting as a facioscapulohumeral syndrome. Abnormalities of nerve conduction present in 3 clinically affected cases were attributed to co-existent diabetes mellitus. Quantitative CNEMG showed a reduction of the mean motor unit potential duration and increased incidence of polyphasic potentials in all 5 clinically manifest cases consistent with a primary myopathic disorder. Similar but less marked changes were found in 6 of the clinically asymptomatic individuals revealing the presence of a subclinical myopathy. Abnormalities on SFEMG consisting of increases in fibre density and/or jitter were present in all the clinically affected and in 5 clinically normal cases. These changes indicate local reorganization of the spatial arrangement of muscle fibres of the motor unit and a disturbance of neuromuscular transmission. The CNEMG and SFEMG findings are discussed in relation to the histopathological changes in 4 cases.