A boy with mitochondrial cytopathy and neuropathological changes of subacute necrotizing encephalopathy is reported. Conditions with abnormal mitochondria in muscle and/or brain are reviewed and the role of mitochondrial abnormalities in spongiform encephalopathies is discussed. It is suggested that electronmicroscopical and histochemical investigations of muscle tissue would be of value in patients with Leigh's syndrome or other forms of spongiform encephalopathies.