Leber's hereditary optic neuropathy (LHON) is a maternally inherited disease caused by mitochondrial DNA (mtDNA) mutations. In this study, the mtDNA/nuclear DNA ratio was evaluated in 11 LHON patients with the 14484 mutation, 13 asymptomatic carriers and 18 non-carrier relatives as controls, to reveal possible relationships between the disease and mtDNA content. DNAs from peripheral blood lymphocytes were subjected to quantitative PCR. Gender differences and age-dependent changes in the mtDNA content were not observed. Significant increase in the mtDNA content was observed only in the asymptomatic carriers (P<0.05). This indicated that individuals whose mtDNA content had increased and been maintained at certain levels were free from LHON development, whereas those whose levels had not, had developed LHON. Since the asymptomatic carriers are the stock of the future LHON patients, monitoring the mtDNA content in patients and their relatives may help to predict the prognosis of the disease.