Ageing is a risk factor for cardiovascular disease. This study showed that microRNA-34a (miR-34a) expression is induced in the ageing mouse heart and contributes to the decline in cardiac function. Genetic inhibition of miR-34a reduced age- and myocardial infarction-associated cardiomyocyte death and fibrosis. The authors showed that the target of miR-34a was protein phosphatase 1 regulatory subunit 10 (PPP1R10), which is involved in apoptosis and DNA repair. Overexpression of this target reduced telomere shortening, DNA damage responses and apoptosis in cardiomyocytes. In mice, genetic inhibition of miR-34a caused upregulation of PPP1R10 in the heart and reduced cardiac DNA damage after acute myocardial infarction.