A mother and two of her daughters had deafness and cortical reflex myoclonus; the mother also had mild truncal ataxia. Muscle and skin biopsy specimens revealed abundant ragged-red fibres and abnormal mitochondria. The son of one of the daughters had sensorineural deafness. Three other grandchildren were asymptomatic. The two daughters also had diabetes mellitus, hypertension and cardiomyopathy. Another daughter died of renal failure. The mother lost her hearing in her 70s, one daughter in her 30s, and the other daughter and the grandson in their 20s. The mother has had transient episodes (24-48 hours) of temporal disorientation, severe action myoclonus, and ataxia for about eight years. This is the first reported family with inherited deafness, myoclonus, and ataxia with mitochondrial pathology.