Platelets play a role in the pathogenesis of ST-segment elevation myocardial infarction (STEMI). We assessed the relationship between mean platelet volume (MPV) on admission and pre- and postinterventional flow with the infarct-related artery (IRA) in patients with STEMI. We prospectively included 840 patients with STEMI who underwent primary percutaneous coronary intervention (PCI). The patients were divided into 3 groups according to MPV tertiles. Pre- and post-PCI Thrombolysis In Myocardial Infarction (TIMI) flow grade was determined. Initial TIMI flow grade 3 was accepted as patent IRA. After the primary PCI, normal flow was defined as post-PCI TIMI flow 3. When the MPV was increased, the incidence of pre-PCI patent IRA (P =.004) and post-PCI normal TIMI flow (P <.001) was significantly decreased. Multivariate analysis showed that MPV was independently associated with post-PCI TIMI flow grade.