OBJECTIVE: To quantify the association between exposure to higher temperatures and the risk of myocardial infarction at an hourly temporal resolution. DESIGN: Case-crossover study. SETTING: England and Wales Myocardial Ischaemia National Audit Project (MINAP) database. PARTICIPANTS: 24,861 hospital admissions for myocardial infarction occurring in 11 conurbations during the warmest months (June to August) of the years 2003-09. MAIN OUTCOME MEASURE: Odds ratio of myocardial infarction for a 1 °C increase in temperature. RESULTS: Strong evidence was found for an effect of heat acting 1-6 hours after exposure to temperatures above an estimated threshold of 20 °C (95% confidence interval 16 °C to 25 °C). For each 1 °C increase in temperature above this threshold, the risk of myocardial infarction increased by 1.9% (0.5% to 3.3%, P=0.009). Later reductions in risk seemed to offset early increases in risk: the cumulative effect of a 1 °C rise in temperature above the threshold was 0.2% (-2.1% to 2.5%) by the end of the third day after exposure. CONCLUSIONS: Higher ambient temperatures above a threshold of 20 °C seem to be associated with a transiently increased risk of myocardial infarction 1-6 hours after exposure. Reductions in risk at longer lags are consistent with heat triggering myocardial infarctions early in highly vulnerable people who would otherwise have had a myocardial infarction some time later ("short term displacement"). Policies aimed at reducing the health effects of hot weather should include consideration of effects operating at sub-daily timescales.