The high frequency of mitochondrial DNA mutation at the nucleotide position (nt) 11,778 was reported in cases of Leber's hereditary optic neuropathy (LHON) after the first report by Wallace et al.. We already reported that it provided a simple diagnostic test by means of PCR (polymerase chain determined the diagnosis of LHON in a case. No nt 11,778 mutation was found in patients with the other optic nerve diseases and in normal controls. This shows the usefulness of molecular diagnosis in LHON. Problems of genetic counselling for patients and female carriers and the possibilities to clarify the cause of LHON were discussed.