['Bivalirudin reduces bleeding events and is associated with a lower mortality than the combination of unfractionated heparin (UFH) and glycoprotein IIb/IIIa inhibitor during primary percutaneous coronary intervention (PCI). However, the effect of adding UFH in patients with ST elevation myocardial infarction (STEMI) treated with bivalirudin during primary PCI is unknown.', 'Patients enrolled in the national Swedish Coronary Angiography and Angioplasty Registry who underwent primary PCI due to STEMI with bivalirudin as anticoagulant were evaluated. Patients were divided into two groups: those treated with bivalirudin only and those treated with bivalirudin plus a bolus dose of UFH.', '2996 patients were included in the study: 1928 (64%) received only bivalirudin and 1068 (36%) received bivalirudin plus a bolus dose of UFH. The primary combined endpoint of death or target lesion thrombosis at 30 days occurred more often in the bivalirudin group (11.3% vs 6.5%, OR 0.55, 95% CI 0.41 to 0.72, p<0.001). This difference remained significant after adjustment (HR 0.64, 95% CI 0.44 to 0.95, p=0.03). Death at 30 days and definite target lesion thrombosis at 30 days did not differ between the two groups after adjustment (9.2% vs 5.1%, adjusted HR 0.66, 95% CI 0.42 to 1.03, p=0.07 and 2.3% vs 1.5%, adjusted HR 0.59, 95% CI 0.27 to 1.33, p=0.21, respectively).'] Is unfractionated heparin administration in patients treated with bivalirudin during primary percutaneous coronary intervention associated lower mortality and target lesion thrombosis : a report from the Swedish Coronary Angiography and Angioplasty Registry ( SCAAR )?