AIMS: It is widely thought that ST-elevation myocardial infarction (STEMI) is more likely to occur without warning (i.e. an unanticipated event in a previously healthy person) than non-ST-elevation myocardial infarction (NSTEMI), but no large study has evaluated this using prospectively collected data. The aim of this study was to compare the evolution of atherosclerotic disease and cardiovascular risk between people going on to experience STEMI and NSTEMI. METHODS: We identified patients experiencing STEMI and NSTEMI in the national registry of myocardial infarction for England and Wales (Myocardial Ischaemia National Audit Project), for whom linked primary care records were available in the General Practice Research Database (as part of the CALIBER collaboration). We compared the prevalence and timing of atherosclerotic disease and major cardiovascular risk factors including smoking, hypertension, diabetes, and dyslipidaemia, between patients later experiencing STEMI to those experiencing NSTEMI. RESULTS: A total of 8174 myocardial infarction patients were included (3780 STEMI, 4394 NSTEMI). Myocardial infarction without heralding by previously diagnosed atherosclerotic disease occurred in 71% STEMI (95% CI 69-72%) and 50% NSTEMI patients (95% CI 48-51%). The proportions of myocardial infarctions with no prior atherosclerotic disease, major risk factors, or chest pain was 14% (95% CI 13-16%) in STEMI and 9% (95% CI 9-10%) in NSTEMI. The rate of heralding coronary diagnoses was particularly high in the 12 months before infarct; 4.1-times higher (95% CI 3.3-5.0) in STEMI and 3.6-times higher (95% CI 3.1-4.2) in NSTEMI compared to the rate in earlier years. CONCLUSIONS: Acute myocardial infarction occurring without prior diagnosed coronary, cerebrovascular, or peripheral arterial disease was common, especially for STEMI. However, there was a high prevalence of risk factors or symptoms in patients without previously diagnosed disease. Better understanding of the antecedents in the year before myocardial infarction is required.