BACKGROUND The comparative efficacy of various anticoagulation strategies has not been clearly established in patients with acute myocardial infarction who are undergoing percutaneous coronary intervention (PCI) according to current practice, which includes the use of radial-artery access for PCI and administration of potent P2Y 12 inhibitors without the planned use of glycoprotein IIb/IIIa inhibitors. METHODS In this multicenter, randomized, registry-based, open-label clinical trial, we enrolled patients with either ST-segment elevation myocardial infarction (STEMI) or non-STEMI (NSTEMI) who were undergoing PCI and receiving treatment with a potent P2Y(12) inhibitor (ticagrelor, prasugrel, or cangrelor) without the planned use of glycoprotein IIb/IIIa inhibitors. The patients were randomly assigned to receive bivalirudin or heparin during PCI, which was performed predominantly with the use of radial-artery access. The primary end point was a composite of death from any cause, myocardial infarction, or major bleeding during 180 days of follow-up. RESULTS A total of 6006 patients (3005 with STEMI and 3001 with NSTEMI) were enrolled in the trial. At 180 days, a primary end-point event had occurred in 12.3% of the patients (369 of 3004) in the bivalirudin group and in 12.8% (383 of 3002) in the heparin group (hazard ratio, 0.96; 95% confidence interval [CI], 0.83 to 1.10; P = 0.54). The results were consistent between patients with STEMI and those with NSTEMI and across other major subgroups. Myocardial infarction occurred in 2.0% of the patients in the bivalirudin group and in 2.4% in the heparin group (hazard ratio, 0.84; 95% CI, 0.60 to 1.19; P = 0.33), major bleeding in 8.6% and 8.6%, respectively (hazard ratio, 1.00; 95% CI, 0.84 to 1.19; P = 0.98), definite stent thrombosis in 0.4% and 0.7%, respectively (hazard ratio, 0.54; 95% CI, 0.27 to 1.10; P = 0.09), and death in 2.9% and 2.8%, respectively (hazard ratio, 1.05; 95% CI, 0.78 to 1.41; P = 0.76). CONCLUSIONS Among patients undergoing PCI for myocardial infarction, the rate of the composite of death from any cause, myocardial infarction, or major bleeding was not lower among those who received bivalirudin than among those who received heparin monotherapy. (Funded by the Swedish Heart-Lung Foundation and others;