Patients with mitochondrial disease may present to the Intensive Care Unit (ICU) with a variety of neurological and general medical disorders. Eleven patients were admitted to a neurological ICU between 1970 and 1992 because of respiratory insufficiency, status epilepticus and/or metabolic encephalopathy associated with mitochondrial disease. Respiratory impairment occurred in eight patients and was associated with nocturnal hypoventilation due to respiratory muscle weakness, aspiration due to bulbar weakness and abnormalities of central control leading to a reduced CO2 drive, irregular respiratory patterns and sleep apnoea. Seven patients received continuous respiratory support during the acute illness; three were subsequently weaned to domiciliary ventilation, and four died. Five patients had stroke-like episodes, which in two were recurrent. Four patients developed tonic-clonic grand mal epilepsy associated with myoclonic fits (2 patients), absences (2), focal fits (1) and status epilepticus (2). Encephalopathy was associated with recurrent lactic acidosis (2 patients), cardiac failure (2), hyponatraemia (2), renal abnormalities (3) and complete heart block (1). Although rare, mitochondrial disease should be considered in any patient with unexplained respiratory failure, intractable epilepsy, lactic acidosis or recurrent stroke.