Leber's hereditary optic neuropathy (LHON), a maternally inherited disease causing severe bilateral visual loss in young men, is linked to 12 point mutations in mitochondrial DNA, the most common of which is at the nucleotide position 11778. The 11778 point mutation has also been detected in several patients with possible multiple sclerosis (MS), especially women with severe visual loss in both eyes. Because frequent and severe optic neuropathy is a feature of MS in Japan, we screened 80 Japanese MS patients for the presence of the 11778 mutation by mutation-specific polymerase chain reaction. Eighteen women with MS had bilateral optic neuropathy, but none had the mutation at 11778. There is no association between Japanese MS and the 11778 mitochondrial DNA mutation.