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Purpose: Single large-scale mitochondrial DNA deletions account for a quarter of mitochondrial disease cases and occur sporadically with unknown risk factors. Mitochondrial DNA deletions accumulate with age in many tissues. Primordial germ cells, the precursors of oocytes are made by our grandmothers, therefore we wanted to determine whether age of maternal grandmother is a risk factor for sporadic mitochondrial DNA deletions., Methods: Twenty-nine patients with sporadic single mitochondrial DNA deletions from the Newcastle UK cohort provided dates of birth for mothers and maternal grandmothers plus father and paternal grandmother (healthy controls)., Results: Mean age for grandmothers at birth of a mother of an affected patient was 28.5 years (SD +/- 6.9) for single mitochondrial DNA deletions maternal grandmothers and 28.2 years (SD +/- 6.1) for healthy control paternal grandmothers., Conclusion: Maternal grandmother age is not a risk factor for sporadic mitochondrial DNA deletions, an important observation in a population where many women are delaying reproduction.