We tested the efficacy of coenzyme Q10 (ubidecarenone, CoQ10) therapy in patients with Kearns-Sayre syndrome and other mitochondrial myopathies with chronic progressive external ophthalmoplegia (CPEO). We treated seven patients for 1 year with daily oral administration of 120 mg of CoQ10. Throughout the treatment most of our patients showed a progressive reduction of serum lactate and pyruvate levels following standard muscle exercise and generally improved neurologic functions. The ECG and echocardiogram showed no significant changes in our patients. None of our patients showed any improvement in ptosis and CPEO.