The observation of a multiple sclerosis (MS)-like illness in patients, particularly women, who carry the most common Leber's hereditary optic neuropathy mitochondrial DNA (mtDNA) mutation may indicate a contributory role for mitochondrial genes in genetic susceptibility to MS. We screened 307 unrelated MS patients, ascertained from population surveys, for the pathogenic Leber's hereditary optic neuropathy mutations at positions 11778 and 3460 of mtDNA, and also studied 20 patients with prominent and early optic nerve involvement. In addition, these 307 patients and 129 control subjects were investigated for the base change at position 13708, which has been suggested to play a role in the pathogenesis of Leber's hereditary optic neuropathy. Neither of the pathogenic mtDNA mutations occurred in the unselected MS patients. The 13708 base change was present in MS patients at a frequency similar to that in healthy control subjects. Three of the patients selected on the basis of severe optic nerve involvement had either the 11778 (one) or 3460 (two) mutations associated with Leber's hereditary optic neuropathy. All were women and none had affected relatives. We conclude that these mutations do not contribute to genetically determined susceptibility in typical MS patients, although a mitochondrial genetic component in the etiology of MS remains possible. A subgroup of MS patients, particularly females with severe bilateral visual failure due to optic neuropathy, may harbor a Leber's hereditary optic neuropathy mutation and we suggest that mtDNA analysis is appropriate in these patients.