Objective: Atherosclerotic plaque rupture leading to coronary artery occlusion is the culprit event which underpins a majority of acute myocardial infarction, and Matrix metalloproteinases (MMPs) contribute to atherosclerotic plaque rupture by involving in extracellular matrix degradation and artificial balloon extrusion. Patients with ST-segment elevation myocardial infarction (STEMI) caused by plaque rupture are at high risk for slow reflow, but the relationship between reflow and MMP-9 remains unclear, especially in the real world of local plaque rupture. We investigated the association between the levels of matrix metalloproteinase-9 (MMP-9) in infarct-related arterial infusion and the risk of slow reflow in STEMI patients following percutaneous coronary intervention (PCI). Methods: 65 eligible acute STEMI patients undergoing successful PCI were included in the current study. Blood samples were obtained from the extraction catheter placed distal to the lesion during PCI. Plasma MMP-9 levels were determined by immunoassay method. Results: Using multiple logistic regression analysis, MMP-9 levels (OR 0.881, CI 0.791-0.981; P=0.021) was found to be a significant risk factor of slow flow together with HSCORE (OR=0.085, CI 0.014-0.506; P=0.007). ROC curve with area under the curve (0.740) and 95% confidence interval (0.607-0.872) revealed that lesion MMP-9 changes had an predictive value for no reflow(P=0.002). Conclusions:The present study indicated that plasma MMP-9 levels in the culprit coronary artery was associated with slow flow in patients with ST-elevation MI following successful primary PCI.