A 42-year-old woman was admitted for complaining of gait disturbance lasting for 2 years. She had a past history of hearing loss and bilateral ocular ptosis since her age of 21 years. Neurological examinations revealed severe external ophthalmoplegia, ptosis and muscular weakness of neck and extremities, but no dementia. Laboratory examinations showed high level of serum CK and slight elevations of CSF protein, lactate and pyruvate. The QT interval on ECG was prolonged. The EEG finding was slightly abnormal. The neurogenic pattern on EMG was noted. Muscle biopsy was performed at the quadriceps muscle. Its specimens demonstrated mixed changes of muscular atrophy and ragged red fibers. From these findings, Kearns-Sayre-Shy syndrome was diagnosed. It was noted in this case that brain MRI (T2-weighted image) revealed bilateral diffuse high signal intensity areas on cerebral white matter and brainstem. It seems that the lesions of central nervous system may progress subclinically for many years in cases of KSS, as our case shows.