Background: Hospital mortality after acute myocardial infarction (AMI, ICD-10: I21–I22) is used as OECD indicator of the quality of acute care. The reported AMI hospital mortality in Germany is more than twice as high as in the Netherlands or Scandinavia. Yet, in Europe, Germany ranks high in health spending and availability of cardiac procedures. We provide insights into this contradictory situation. Methods: Information was collected on possible factors causing the reported differences in AMI mortality such as prevalence of risk factors or comorbidities, guideline conform treatment, patient registration, and health system structures of European countries. International experts were interviewed. Data on OECD indicators ‘AMI 30-day mortality using unlinked data’ and ‘average length of stay after AMI’ were used to describe the association between these variables graphically and by linear regression. Results: Differences in prevalence of risk factors or comorbidities or in guideline conform acute care account only to a smaller extent for the reported differences in AMI hospital mortality. It is influenced mainly by patient registration rules and organization of health care. Non-reporting of day cases as patients and centralization of AMI care—with more frequent inter-hospital patient transfers—artificially lead to lower calculated hospital mortality. Frequency of patient transfers and national reimbursement policies affect the average length of stay in hospital which is strongly associated with AMI hospital mortality (adj R ² = 0.56). AMI mortality reported from registries is distorted by different underlying populations. Conclusion: Most of the variation in AMI hospital mortality is explained by differences in patient registration and organization of care instead of differences in quality of care, which hinders cross-country comparisons of AMI mortality. Europe-wide sentinel regions with comparable registries are necessary to compare (acute) care after myocardial infarction. Graphical abstract: [Figure not available: see fulltext.].