Aim To study the relationships between carotid plaque and plasma matrix metalloproteinase and tissue inhibitor metalloproteinase in patients with acute coronary syndrome(ACS). Methods Using high-resolution ultrasonic instrument the intima-media thickness(IMT)and plaque score in carotid artery were detected in 30 patients with ACS,29 patients with stable angina pectoris(SAP)and 17 control subjects,and the plasma concentration of matrix metalloproteinase9(MMP-9)and tissue inhibitor metalloproteinase-1(TIMP-1)were measured by sandwich enzyme immunoassay technique. Results Compared with SAP group and control group,plasma MMP-9,TIMP-1,MMP-9/TIMP-1,and IMT,whole plaque score,soft plaque score and hard plaque score in carotid artery in ACS group were significantly increased(p<0.01).MMP-9 had positive relationship with IMT,whole plaque score and soft plaque score in carotid artery(r=0.468,p<0.01;r=0.592,p<0.01;r=0.677,p<0.01),respectively.TIMP-1 had positive relationship with IMT,whole plaque score,soft plaque score and hard plaque score in carotid artery(r=0.449,p<0.01;r=0.493,p<0.01;r=0.435,p<0.01;r=0.227,p<0.05),respectively.MMP-9/TIMP-1 had positive relationship with IMT,whole plaque score and soft plaque score in carotid artery(r=(0.253),p<0.05;r=0.431,p<0.01;r=0.547,p<0.01), respectively.Removing age factor,partial correlation analysis showed IMT had positive relationship with MMP-9(r=0.461,p<0.001),TIMP-1(r=0.441,p<0.001),MMP-9/TIMP-1(r=0.241,p<0.01),respectively. Conclusions Plasma MMP-9,TIMP-1,MMP-9/TIMP-1,and IMT,whole plaque score,soft plaque score and hard plaque score in carotid artery in ACS group are significantly higher than those in SAP group and control group.Plasma MMP-9,TIMP-1,MMP-9/TIMP-1 have positive relationship with IMT,whole plaque score and soft plaque score in carotid artery,respectively.