Abstract: AIM:To detect the changes of mean platelet volume (MPV) in patients with acute ST-segment elevation myocardial infarction (STEMI) and its association with coronary angiograms. METHODS: A total of 200 patients with STEMI who underwent emergency coronary angiography were enrolled. Admission MPV was measured and cardiac ultrasound examinations were conducted in the first 24 h. Gensini coronary score and infarct-related artery (IRA) flow before and after primary percutaneous coronary intervention (PPCI) were calculated. Two hundred in-hospital patients with non-CAD were used as controls. RESULTS: MPV significantly increased in STEMI patients compared with that in the control group. After adjustment, MPV was negatively correlated with HDL-C and left ventricular ejection (LVEF) but was positively correlated with Gisini coronary score. MPV in multiple lesions subgroup was significantly higher than in the single lesion subgroup and higher in the left anterior descending coronary artery as IRA subgroup than in the left circumflex artery as IRA subgroup. If angiographic IRA flow reached TIMI grade 1-3 before PPCI, it was defined as spontaneous reperfusion. Patients without spontaneous reperfusion had markedly increased MPV compared with patients with spontaneous reperfusion. Patients whose IRA flow was less than TIMI grade 3 after PPCI had significantly elevated MPV compared with patients of TIMI grade 3. CONCLUSION: MPV is significantly increased in STEMI patients and MPV is associated with severity of coronary lesions and slow IRA flow.