OBJECTIVES: This study sought to determine clinical, procedural, and treatment factors associated with acute stent thrombosis (AST) in the EUROMAX (European Ambulance Acute Coronary Syndrome Angiography) trial. BACKGROUND: Bivalirudin started during transport for primary percutaneous coronary intervention (PCI) for ST-segment elevation myocardial infarction significantly reduced major bleeding compared with heparin with or without glycoprotein IIb/IIIa inhibitors (GPI), but it was associated with an increase in AST. METHODS: We compared patients with (n = 12) or without AST (n = 2,184) regarding baseline, clinical, and procedural characteristics and antithrombotic treatment strategies (choice of P2Y12 inhibitor, post-primary PCI bivalirudin infusion dose [0.25 mg/kg/h, or BIV-LOW] vs. [1.75 mg/kg/h, or BIV-PCI] vs. heparin ± GPI). Logistic regression was performed to identify independent correlates of AST. RESULTS: The overall AST rate was 0.6% and was higher with bivalirudin than with heparin ± GPI (1.1% vs. 0.2%; p = 0.007). Median time to AST was 2.3 h (interquartile range: 1.9 to 2.8 h). Patients with AST had less hypertension (2 of 14 [14.0%] vs. 961 of 2,182 [44.0%]; p = 0.03), and more frequently received GPI (11 of 14 [78.6%] vs. 880 of 2,183 [40.3%]; p = 0.004). Multivariate analysis using Firth penalized maximum likelihood estimation found hypertension (odds ratio [OR]: 0.24, 95% confidence interval [CI]: 0.07 to 0.92; p = 0.037) and BIV-LOW (OR: 5.8, 95% CI: 1.5 to 22.2; p = 0.010) predictive of AST. Choice of P2Y12 inhibitor had no impact on AST. Compared with heparin ± GPI, AST rates were higher for BIV-LOW (11 of 670 [1.6%] vs. 2 of 947 [0.2%]; p = 0.008), but not different for BIV-PCI (1 of 244 [0.4%]; p = 0.588). CONCLUSIONS: In this post-hoc analysis from EUROMAX, AST occurred very early and was not mitigated by the novel P2Y12 inhibitors. Prolonging the bivalirudin infusion at the PCI dose (but not at a lower dose) appeared to mitigate the risk of AST.