Eight women are described who presented with bilateral, usually sequential, optic neuropathy, six of whom later developed a neurological syndrome indistinguishable from multiple sclerosis (MS). Magnetic resonance imaging, performed in five of the patients with an MS-like illness and in the two others with optic neuropathy alone, showed widespread white matter lesions as seen in MS. All of these women had matrilineal relatives with Leber's hereditary optic neuropathy, although this was not always apparent at presentation, and the most common mitochondrial DNA mutation associated with this disorder was detected in each of the women and their affected relatives. On the basis of observations made in these patients, the clinical features of Leber's hereditary optic neuropathy in males, and evidence for mitochondrially encoded peptides involved in the immune response in rodents, we propose that optic nerve damage in this disease could be immunologically mediated and that mitochondrial genes may contribute to susceptibility to MS.