Objective: Platelet-derived growth factor (PDGF) induces matrix metalloproteinase (MMP), which is regarded as a biomarker of plaque rupture or vulnerability. The aim of this study is to investigate those interactions in human coronary arteries at the onset of ST-segment elevation myocardial infarction (STEMI).Methods: Thirty-two patients with STEMI who underwent primary percutaneous coronary intervention (PCI) were enrolled in this study. Plasma levels of PDGF-BB and MMP-9 were measured from infarct-related artery (IRA) and from femoral artery (FA) during PCI.Results: Plasma levels of PDGF-BB and MMP-9 in the IRA were significantly higher than those in the FA (PDGF-BB: median 3130 pg/ml, IQR (interquartile range): 2020 to 4375 pg/ml vs. median 2605 pg/ml, IQR: 1305 to 3290 pg/ml, p <0.01, MMP-9: median 49 ng/ml, IQR: 35 to 100 ng/ml vs. median 42 ng/ml, IQR: 27 to 78 ng/ml, p = 0.04, IRA and FA, respectively).Conclusions: This in vivo study demonstrated that PDGF-BB with MMP-9 seems to play a role in coronary plaque instability in acute phase of STEMI.