The neutrophil-to-lymphocyte ratio (NLR) has been proven to be reliable inflammatory marker for atherosclerotic process and predictor for clinical outcomes in patients with various cardiovascular diseases. Recent study reported elevated NLR was associated with impaired myocardial perfusion in ST-segment elevation myocardial infarction (STEMI) patients. The investigators sought to determine whether NLR is associated with coronary microcirculation assessed by index of microcirculatory resistance (IMR) in STEMI patients who undergone primary percutaneous coronary intervention (PCI). A total of 123 patients with STEMI underwent successful primary PCI were consecutively enrolled. NLR at admission was calculated, and the patients were divided into three groups according to NLR tertiles. IMR was measured by intracoronary thermodilution-derived method immediately after index PCI.