The authors report a 29-year-old woman with marked atrophy of the cerebellum, medulla oblongata, and spinal cord, dementia, diffuse white matter abnormality on MRI, ragged-red fibers, and R88C mutation in the human glial fibrillary acidic protein (GFAP). Mitochondria DNA (mtDNA) analysis showed a rare polymorphism at A8291G. This mtDNA polymorphism, which has been associated with limb-girdle type mitochondrial myopathy, may modify the clinical symptoms of this juvenile form of Alexander disease with GFAP mutation.