Objectives The aim of this study was to determine whether response to prasugrel is associated with the proportion of circulating reticulated platelets (RPs) in patients with ST-segment elevation myocardial infarction (STEMI). Background Despite better pharmacodynamic properties and clinical efficacy of prasugrel compared with clopidogrel, antiplatelet responses to prasugrel are not uniform. The mechanism of this variability in response is not clear. RPs, young hyperactive forms, are increased during situations of enhanced platelet turnover. Methods Patients with STEMI treated with primary percutaneous intervention (PCI) and prasugrel were tested for platelet reactivity using purinergic receptor P2Y, G-protein coupled, 12 (P2Y12) assay and multiple electrode aggregometry (MEA). RP levels were determined using flow cytometry with thiazole orange staining. Tests were performed at 2 to 4 days and 30 days post-PCI. Platelet function was compared by varying levels of RPs, analyzed as continuous (regression analysis) and categorical (tertiles) variables. Results Sixty-two patients were included (mean age: 57.5 ± 8 years; 21.2% women; 27.7% diabetes). At the early time point, RP levels were strongly correlated with platelet reactivity when evaluated by the P2Y12 assay (Spearman's correlation coefficient: 0.55 for P2Y12 reaction units, –0.49 for percent inhibition) and MEA (Spearman's: 0.50). The upper tertile of RPs displayed higher platelet reactivity compared with the middle and lower tertiles, according to P2Y12 assay and MEA. Similar results with strong correlations between RP and platelet reactivity were noted at 30 days post-PCI. Conclusions The proportion of circulating RPs strongly correlates with response to prasugrel in patients with STEMI treated with PCI. High levels of RPs are associated with increased platelet reactivity despite prasugrel treatment.