Background Inflammation within vulnerable coronary plaques may cause unstable angina by promoting rupture and erosion. C-reactive protein (CRP) is the most reliable and accessible test method for clinical use for identifying coronary artery disease event. Matrix metalloproteinase 9 (MMP-9) is highly over-expressed in the vulnerable regions of a plaque.Our aim was to evaluate the plasma levels of MMP-9 and hsCRP in subjects with both unstable angina and coronary plaques, as well as in those with unstable angina without coronary plaques.Methods Patients with newly diagnosed unstable angina pectoris from clinical presentation and ECG, who were undergoing coronary angiography from April 2007 to April 2009, were included in this study. A total of 170 subjects were enrolled in the study. Before angiography, the baseline clinical data (mainly including conventional risk factors) was collected.These patients were divided into two groups, a non-plaque group (G1) which included 55 patients with no significant stenosis or less than 20% stenosis in at least one of the major coronary artery branches, and a plaque group (G2) which included 115 patients with at least one of the major coronary artery branches unstable angina pectoris with at least 50% stenosis of one major coronary artery. The patients presenting with calcified nodules of a major coronary artery were excluded from this study.We examined the serum levels of MMP-9 for all cases by multi-effect enzyme-linked immunosorbent assay.Results There was a significant difference in the serum levels of MMP-9 between the two groups (P＜0.001). The percentage of patients with hypertension, diabetes and current smokers were significantly different between the two groups (P=0.034, P=0.031, and P=0.044 respectively). The univariate Logistic regression analyses of risk factors showed that smoking was the main risk factor for angina in the non-plaque group with the OR being 1.95 (95% Cl 1.02-3.75).Hypertension, diabetes mellitus were negatively related with the occurrence of angina in the non-plaque group with the ORs being 0.50, and 0.36, respectively (95% Cl 0.26-0.96 and 0.14-0.94). The MMP-9 level was negatively related to the occurrence of angina in the non-plaque group with an OR of 0.59 (95% Cl 0.47-0.81).Conclusions There is a significantly difference in MMP-9 levels between the plaque and non-plaque groups. Current smoking has a significant influence on unstable angina patients without documented plaques. The serum MMP-9 level may be a significant biomarker which can help differentiate patients with unstable angina with plaques from those with unstable angina but without plaques.