Leber's hereditary optic neuropathy is a maternally inherited disorder most commonly associated with a mitochondrial DNA mutation at nucleotide position 11778. We report four patients, including a man and a black woman, with the 11778 mutation, who have optic neuropathy and clinical or paraclinical evidence of demyelinating disease. These data support an association of this mitochondrial DNA mutation with demyelinating disease that has a marked female predominance.