Myoclonic epilepsy associated with ragged red fibers (MERRF) syndrome is one of the major mitochondrial encephalomyopathies, with the involvement of various organs, which could be caused by mitochondrial A8344G DNA mutation. Monostotic fibrous dysplasia of bone, an asymptomatic developmental disorder, was reported to result from c-fos overexpression in osteogenic cells. Mitochondrial A8344G mutation has been shown to increase c-fos expression in a MERRF cybrid cell line. The authors describe a boy aged 10 years and 2 months with MERRF syndrome and A8344G mutation. Visual disturbance developed and deteriorated rapidly 5 months after the diagnosis of MERRF. A brain magnetic resonance imaging revealed optic nerve compression by sphenoid fibrous dysplasia, which was confirmed by histology. Fibrous dysplasia has never been mentioned in MERRF patients in the literature. This rare association may be because of underestimation, or it could be a coincidence. Care should be taken to explore the skeletal system in MERRF patients with focal symptoms.