The chronological accumulation of mitochondrial DNA mutations has been proposed as a potential mechanism in the physiological processes of ageing and age‐related disease. We discuss the evidence behind this theory and relate some of the ageing mitochondrial changes to mitochondrial DNA disorders. In particular, we describe the aggregation of cytochrome c oxidase‐deficient cells in both skeletal muscle and the CNS in normal ageing as seen in the mitochondrial DNA disorders. These mitochondrial enzyme‐deficient cells have been shown to occur in significant quantities in both muscle and CNS in patients with mitochondrial DNA disorders. In both ageing and mtDNA disorder muscle these cytochrome c‐deficient fibres contain high levels of a single mutant strain of Mitochondrial DNA. Whether these mutations are a primary or secondary event in the physiology of ageing remains to be determined.