We describe three patients with mitochondrial myopathy, dementia, loss of vision and hearing, seizure disorder with myoclonus, intermittent headaches of a vascular type, visual hallucinations, cerebellar dysfunction, and lactic acidosis. Muscle biopsies in all patients and liver biopsy in one revealed abnormal mitochondria. The disorder may be due to a deficiency of mitochondrial NADH-CoQ dehydrogenase.