In the randomized rapid intravascular cooling in myocardial infarction as adjunctive to percutaneous coronary intervention (RAPID MI-ICE) and rapid endovascular catheter core cooling combined with cold saline as an adjunct to percutaneous coronary intervention for the treatment of acute myocardial infarction CHILL-MI studies, hypothermia was rapidly induced in conscious patients with ST-elevation myocardial infarction (STEMI) by a combination of cold saline and endovascular cooling. Twenty patients in RAPID MI-ICE and 120 in CHILL-MI with large STEMIs, scheduled for primary percutaneous coronary intervention (PCI) within <6 hours after symptom onset were randomized to hypothermia induced by rapid infusion of 600-2000 mL cold saline combined with endovascular cooling or standard of care. Hypothermia was initiated before PCI and continued for 1-3 hours after reperfusion aiming at a target temperature of 33°C. The primary endpoint was myocardial infarct size (IS) as a percentage of myocardium at risk (IS/MaR) assessed by cardiac magnetic resonance imaging at 4±2 days. Patients randomized to hypothermia treatment achieved a mean core body temperature of 34.7°C before reperfusion. Although significance was not achieved in CHILL-MI, in the pooled analysis IS/MaR was reduced in the hypothermia group, relative reduction (RR) 15% (40.5, 28.0-57.6 vs. 46.6, 36.8-63.8, p=0.046, median, interquartile range [IQR]). IS/MaR was predominantly reduced in early anterior STEMI (0-4h) in the hypothermia group, RR=31% (40.5, 28.8-51.9 vs. 59.0, 45.0-67.8, p=0.01, median, IQR). There was no mortality in either group. The incidence of heart failure was reduced in the hypothermia group (2 vs. 11, p=0.009). Patients with large MaR (>30% of the left ventricle) exhibited significantly reduced IS/MaR in the hypothermia group (40.5, 27.0-57.6 vs. 55.1, 41.1-64.4, median, IQR; hypothermia n=42 vs. control n=37, p=0.03), while patients with MaR<30% did not show effect of hypothermia (35.8, 28.3-57.5 vs. 38.4, 27.4-59.7, median, IQR; hypothermia n=15 vs. control n=19, p=0.50). The prespecified pooled analysis of RAPID MI-ICE and CHILL-MI indicates a reduction of myocardial IS and reduction in heart failure by 1-3 hours with endovascular cooling in association with primary PCI of acute STEMI predominantly in patients with large area of myocardium at risk. (ClinicalTrials.gov id NCT00417638 and NCT01379261).