The mitochondrial genome has an underdeveloped "DNA repair repertoire" compared with the nuclear genome, making the mitochondrial DNA more susceptible to mutations by endogenous factors such as defects of the mitochondrial polymerase itself, and by exogenous factors such as radiation and UV light. Increased sensitivity to mutagenic factors may account for the mitochondrial DNA polymorphism within ethnic groups and the mitochondrial diseases associated with all mitochondrial DNA mutations, including DNA depletion. The presence in highly developed organisms of a DNA repair repertoire less organized in the mitochondria than in the nuclei might be a source of biologic dysfunction relevant also to aging and cell death. Uncorrected mitochondrial DNA modifications may determine lethal and severe diseases or asymptomatic biochemical dysfunctions. Considering the long life span and the complex metabolism of highly developed cells, the tendency to produce and accumulate mitochondrial DNA mutations may assume a pathogenetic role with aging.