Background: Blood transfusions (BTs) may worsen the prognosis of patients affected by acute coronary syndromes (ACS), although few data detail their impact on short-term events according to clinical presentation (ST Segment Elevation Myocardial Infarction, STEMI vs. Non-ST Segment Elevation ACS, NSTE-ACS). Methods: Patients undergoing percutaneous coronary intervention (PCI) for ACS, with data on BTs, were selected from the BleeMACS registry. The primary end point was the incidence of myocardial infarction during hospitalization (reAMI), the secondary end-points were 30-day mortality and the combined end-point of 30-day mortality and reAMI. Sensitivity analyses were performed according to clinical presentation (STEMI vs. NSTE-ACS). Results: Overall, 13,975 patients were included: mean age was 64.1 years, 10,651 (76.2%) were male and 7711 (55.2%) had STEMI. BTs were administered during hospitalization to 465 (3.3%) patients, who were older and presented a more relevant burden of risk factors. The primary end-point of reAMI occurred in 197 (1.4%) patients,of whom 102 (1.1%) with STEMI. After controlling for confounding variables, BTs independently predicted the primary end-point reAMI in patients admitted for STEMI (OR 4.059, 95% CI 2244-7.344) and not in those admitted for NSTE-ACS. Moreover, BTs independently related to 30-day mortality in STEMI and NSTE-ACS patients and to the composite of 30-day mortality and reAMI in STEMI patients. Conclusions: In patients undergoing PCI for ACS, BTs increase the risk of reAMI only in those admitted for STEMI, and not in those with NSTE-ACS. These results may help physicians to choose appropriate BT administration according to the admission diagnosis. (C) 2016 Elsevier Ireland Ltd. All rights reserved.