Leber's hereditary optic neuropathy (LHON) is a maternally transmitted disease characterized by acute or subacute visual loss predominantly affecting young men. The majority of LHON cases are caused by one of the three primary mitochondrial DNA (mtDNA) mutations: G3460A/ND1, G11778A/ND4, or T14484C/ND6. Although the primary etiological factor of LHON is a mtDNA mutation, the presence of a primary mtDNA mutation does not necessarily lead to visual loss. The pathogenesis of LHON remains unclear. The marked incomplete penetrance and gender bias indicate that additional genetic (nuclear or mitochondrial) and epigenetic factors may also be involved. Deficiency in respiratory chain function and reactive oxygen species (ROS) are believed to play a pivotal role in the pathophysiology of the disease.