OBJECTIVE: In a significant proportion of patients with ST-elevation myocardial infarction (STEMI), microvascular and myocardial reperfusion cannot be regained despite successfully restored thrombolysis in myocardial infarction (TIMI) grade 3 epicardial blood flow. Myocardial blush grade (MBG) is a reliable marker for microvascular patency and predicts short-term and long-term mortality after primary percutaneous coronary intervention (PCI) in patients with acute STEMI, independent of other variables. Mean platelet volume (MPV), a unique measure of platelet size, is an indicator of platelet reactivity. In this study, we aimed to investigate the relation of admission MPV with postinterventional MBG in patients with STEMI and TIMI grade 3 flow at infarct artery after primary PCI. MATERIALS AND METHODS: Three hundred and ten patients were selected as a study group among patients with STEMI and TIMI grade 3 epicardial blood flow after primary PCI. Blood samples for analysis were obtained during the initial evaluation of patients at the emergency department. MBGs of patients were classified at the end of angioplasty. Patients with MBG 0 and 1 were defined as having poor myocardial blush and patients with MBG 2 and 3 were defined as having normal myocardial blush. RESULTS: Patients with poor myocardial blush had higher admission MPV (10.5±1.3 to 9.1±1 fl, P<0.001), higher peak creatine kinase myocardial band isoenzyme levels (260±53 to 190±38 U/l, P<0.001), higher white blood cell count (11.3±4 to 10.3±3×10/μl, P=0.012), and lower left ventricular ejection fraction (42±7 to 51±8%, P<0.001) compared with patients with normal myocardial blush. Linear regression analysis showed that admission MPV was significantly associated with postinterventional MBG (coefficient=0.598, P<0.001). Cardiovascular mortality (13-5%, P=0.013) and acute-subacute stent thrombosis at 3 months (12-8%, P=0.028) were significantly higher in patients with poor postinterventional myocardial blush compared with those with normal myocardial blush. CONCLUSION: The MPV measured at admission is significantly associated with poor postinterventional MBG in patients with STEMI and TIMI grade 3 flow at infarct artery after primary PCI.