Background International research for acute myocardial infarction lacks comparisons of whole health systems. We assessed time trends for care and outcomes in Sweden and the UK. Methods We used data from national registries on consecutive patients registered between 2004 and 2010 in all hospitals providing care for acute coronary syndrome in Sweden and the UK. The primary outcome was all-cause mortality 30 days after admission. We compared effectiveness of treatment by indirect casemix standardisation. This study is registered with ClinicalTrials.gov, number NCT01359033. Findings We assessed data for 119 786 patients in Sweden and 391 077 in the UK. 30-day mortality was 7.6% (95% CI 7.4-7.7) in Sweden and 10.5% (10.4-10.6) in the UK. Mortality was higher in the UK in clinically relevant subgroups defined by troponin concentration, ST-segment elevation, age, sex, heart rate, systolic blood pressure, diabetes mellitus status, and smoking status. In Sweden, compared with the UK, there was earlier and more extensive uptake of primary percutaneous coronary intervention (59% vs 22%) and more frequent use of beta blockers at discharge (89% vs 78%). After casemix standardisation the 30-day mortality ratio for UK versus Sweden was 1.37 (95% CI 1.30-1.45), which corresponds to 11 263 (95% CI 9620-12 827) excess deaths, but did decline over time (from 1.47, 95% CI 1.38-1.58 in 2004 to 1.20, 1.12-1.29 in 2010; p=0.01). Interpretation We found clinically important differences between countries in acute myocardial infarction care and outcomes. International comparisons research might help to improve health systems and prevent deaths.