We report on the age and the causes of death in 16 patients with mitochondrial diseases. Nine patients with mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) died at a mean age of 34 years and three patients with chronic progressive external ophthalmoplegia at a mean age of 56 years. The causes of death were cardiopulmonary failure (n = 5), status epilepticus (n = 4), aspiration pneumonia (n = 2), pulmonary embolism (n = 2), renal failure (n = 1), metabolic disturbance (n = 1), and unknown causes (n = 1). Thus, many patients in this series died of medical complications, some of which may be prevented.