Damaging reactive oxygen species (ROS) are thought to be drivers of mitochondrial mutagenesis and ageing. However, thorough investigation of mitochondrial DNA (mtDNA) mutations has been hampered by a lack of methods to accurately detect such mutations, which are rare. Using a recently developed, highly specific and accurate method called duplex sequencing, the authors assessed mtDNA mutations from brain tissues of old and young individuals. Surprisingly, they found that although the total number of mutations increases with age, there is no increase in mutations that are characteristic of ROS damage, which suggests a rethink of the role of ROS in theories of ageing.