We describe a 32-year-old woman with sequential, severe, painless visual loss in one eye and then the other, and three temporally distinct episodes of neurological disturbance suggestive of demyelination in the spinal cord. She was positive for the T14484C mutation in the mitochondrial genome, one of three common mutations causing Leber's hereditary optic neuropathy. In addition, MRI identified areas of demyelination within the periventricular white matter of the brain and within the spinal cord. The coexistence of multiple sclerosis and Leber's hereditary optic neuropathy (Harding's syndrome) is known to occur more often than would be expected by chance; therefore, screening for the Leber's mutations in multiple sclerosis patients with severe visual loss should be considered because this has important prognostic and genetic implications.