A 9-year-old girl with recently diagnosed juvenile onset diabetes mellitus presented with signs and symptoms of bilateral optic neuropathy. Leber's hereditary optic neuropathy was suspected on the basis of a strong family history. Subsequent mitochondrial DNA testing was positive. Visual recovery occurred once the diabetes was well controlled. This case suggests that such metabolic compromise that occurs in diabetes may precipitate the clinical expression of Leber's optic neuropathy.