Abstract : Introduction: Anticoagulant therapy is critical to prevent ischemic recurrences and complications in acute coronary syndrome (ACS) patients undergoing percutaneous coronary intervention (PCI). Unfractionated heparin (UFH), an injectable anticoagulant has several limitations: lack of predictability of its biological efficacy, platelets activation, heparin-induced thrombopenia and bleedings. Bivalirudin, a synthetic direct thrombin inhibitor has biological properties that promised better clinical outcome in ACS patients undergoing PCI. Areas covered: The present review aimed to summarize two decades of randomized clinical trials that compared bivalirudin to UFH in ACS patients treated with PCI. Early trials highlighted a reduction of bleedings with bivalirudin compared to UFH in combination with glycoprotein inhibitors (GPI). Recent studies questioned this reduction given that GPI are less and less used during PCI. Further, trials raised concerns about the risk of stent thrombosis in patients treated with bivalirudin. In light of this data, bivalirudin has been downgraded in international guidelines and appears as a second line anticoagulant agent after UFH. Expert opinion: The highly questioned reduction of bleedings under bivalirudin and the potential risk of stent thrombosis are unwarranted. Based on clinical trials, UFH has no equivalent in terms of anticoagulation in ACS patients undergoing PCI.