Leber's hereditary optic neuropathy (LHON) is a maternally transmitted disease, characterized by bilateral optic atrophy predominantly in healthy young males. This disorder has shown to be associated with DNA mutation in mitochondrial genome of the patients. We report here a young man who came to the hospital with subacute visual loss in one eye, followed by the other eye within two months. His echocardiogram was normal. A G-->A base substitution at nucleotide position 11,778 which changes a conserved arginine to histidine at amino acid position 340 of ND4, a protein subunit of respiratory chain enzyme complex I in oxidative phosphorylation system, was detected in his leucocyte mitochondrial genome.