A novel mitochondrial DNA C4171A mutation in the ND1 gene in two Korean families with Leber's hereditary optic neuropathy is described. All affected patients recovered spontaneously after suffering months to years of initial visual loss. This mutation replaces leucine with methionine in a conserved extramembrane loop of the ND1 gene and was absent in 514 normal controls and in 63 Leber's hereditary optic neuropathy lineages harboring the primary mutations. We consider mitochondrial DNA C4171A/ND1 a primary causative mutation of Leber's hereditary optic neuropathy with a good prognosis.