© 2014 by Turkish Society of Cardiology.Objective: Mean platelet volume (MPV), one of the indices of platelet reactivity has been shown to be related to impaired angiographic reperfusion in ST-segment elevation myocardial infarction (STEMI) patients treated with primary angioplasty or thrombolytics. However data regarding MPV and its association with ST-segment resolution; an indicator of epicardial and tissue level reperfusion in the setting of STEMI are limited. In this study, we aimed to investigate whether MPV on admission is associated with ST-segment resolution in STEMI patients treated with thrombolytics.Methods: We retrospectively evaluated 232 consecutive patients with a diagnosis of first STEMI who were administered thrombolytic therapy within 12 hours of onset of chest pain. ST segment resolution based on baseline and 90 minute electrocardiographies were measured. Patients were grouped into two as with <50% and ≥50% ST-segment resolution. Admission MPV was measured and compared between two groups.Results: Admission MPV was higher in patients with <50% ST-segment resolution than patients with ≥50% ST-segment resolution (9.9±1.3 fL vs. 8.5±1.1 fL respectively, p<0.001). The receiver operating characteristic analysis yielded a cut-off value of 9.3 fL to predict ST-segment resolution, with sensivity and specifity being 66.7% and 77.9%, respectively. In-hospital mortality rate was high in patients with <50% ST -segment resolution (p=0.002).Conclusion: High MPV on admission in STEMI patients treated with thrombolytics is associated with impaired ST segment resolution.