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In patients with acute myocardial infarction (AMI), the guideline-recommended treatment is mechanical revascularization by percutaneous coronary intervention (PCI), which is effective at reducing mortality. However, a substantial proportion of patients with AMI develop chronic cardiac failure owing to poor restoration of microvascular function and myocardial perfusion, despite restoration of epicardial vessel patency. This occurrence is called the 'no-reflow' phenomenon. Although pathological and clinical observations initially seemed to support the hypothesis that no-reflow was the result of microvascular obstruction, irreversible microvascular injury and subsequent intramyocardial haemorrhage are now also thought to be important factors in this process. Intramyocardial haemorrhage shares several pathophysiological features with the haemorrhagic transformation that occurs after ischaemic stroke. Understanding of the role of intramyocardial haemorrhage in the no-reflow phenomenon and myocardial injury is crucial to the development of new therapeutic strategies to treat AMI. In this Review, we provide a comprehensive overview of the pathogenesis and clinical relevance of intramyocardial haemorrhage, and discuss diagnostic options and future therapeutic strategies.