Investigation of a maternal family of three generations of Leber's hereditary optic neuropathy (LHON) showed four affected and three unaffected individuals. Two of the four patients had recovered near-normal vision, one spontaneously, and one following treatment with idebenone, a quinol compound. One patient whose visual impairment persisted was a heavy consumer of alcohol and tobacco. Molecular genetic analysis of 12 known primary or secondary mutations in mitochondrial DNA (mtDNA) associated with LHON revealed only the 11778 mutation in a homoplasmic fashion with no secondary mutations. The variations in clinical outcome thus could not be explained by synergistically interacting secondary mutations in mtDNA. Environmental factors may play an etiologic role in the development of optic atrophy.