The mitochondrial DNA mutation mt.3243A>G is most commonly associated with maternally inherited diabetes and deafness (MIM 52,000), but it has protean phenotypes including renal disease due to focal segmental glomerulosclerosis. We describe monozygotic twins who both harboured this mutation and developed ESRD. Although otherwise genetically identical, the twins differed in their peripheral blood leucocyte levels of circulating mt.3243A>G heteroplasmy: 20 versus 10%, when assessed at 42 years of age. The twin with the higher heteroplasmy load developed end-stage kidney disease 15 years earlier than her sister. A review of the published literature supports a relationship between heteroplasmy level and the age at the development of the end stage of renal failure in patients with mt.3243A>G-related kidney disease.