Mitochondrial encephalopathies represent a heterogeneous group of various neurological syndromes caused by defects in mitochondrial metabolism. All clinical syndromes can be subdivided by type of biochemical defect into 3 subgroups: defective oxidation, defects in pyruvate metabolism and various defects in the respiratory chain. We present a 12-year-old girl admitted for evaluation of progressive ptosis over a period of 3 years, diagnosed as having the rare mitochondrial encephalopathy, Kearns-Sayre syndrome.