AIMS: To investigate the change of mitochondrial DNA (mtDNA) content in Leber's hereditary optic neuropathy (LHON) with 11778 mutation. METHODS: Mitochondrial DNA content in 27 LHON patients with 11778 mutation, 26 asymptomatic maternal relatives, and 23 normal controls was measured using a competitive polymerase chain reaction (PCR) method. RESULTS: The mean relative content of mtDNA (with respect to the beta actin gene) in LHON patients, asymptomatic maternal relatives, and normal controls was 245.5 (162.3), 238.2 (118.4), and 156.5 (61.6), respectively. There was a statistically significant difference between patients and controls and between relatives and controls. However, no statistically significant difference between patients and unaffected relatives was found. There was no statistically significant difference in the relative content of mtDNA between all males and females carrying 11778 mtDNA mutation CONCLUSION: The results suggest that the increase in mtDNA content in LHON patients with 11778 mtDNA mutation may be due to a compensatory effect for respiratory chain defects of mitochondria. However, the increase of mtDNA content is the result rather than the cause of defective mtDNA. It still cannot explain the pathogenesis of LHON.