Matrix metalloproteinases (MMPs) play a role in collagen breakdown, leading to plaque instability. High levels of MMPs mRNA and proteins, especially MMP-1, MMP-2, MMP-8, MMP-9, and MMP-13, were shown in human atherosclerotic plaques. However, among various MMPs, only MMP-1, MMP-8 and MMP-13, so-called interstitial collagenases, can initiate collagen breakdown. To elucidate whether MMP-1, MMP-8 and MMP-13 levels in blood were high in patients with unstable angina (UAP), we measured serum MMP-1 and plasma MMP-8 and MMP-13 levels in 45 patients with UAP, 175 with stable coronary artery disease (CAD), and 45 controls. Plasma C-reactive protein levels tended to be higher in patients with UAP than in those with stable CAD and controls (median 0.94 vs. 0.69 and 0.51mg/l). Regarding blood levels of MMPs, MMP-13 levels were above the lower detection limit in only one patient with UAP (2%), one with stable CAD (1%), and none in controls. MMP-1 levels did not differ among patients with UAP, stable CAD, and controls (median 4.8, 5.3, and 5.4ng/ml). Notably, MMP-8 levels were higher in patients with stable CAD than in controls (median 3.5ng/ml vs. 2.8ng/ml, P<0.005), however, MMP-8 levels in patients with UAP were much higher than those in stable CAD (3.9ng/ml vs. 3.5ng/ml, P<0.05). In multivariate analysis, MMP-8 level was an independent factor for UAP. Thus, plasma MMP-8 levels were found to be high in patients with UAP, suggesting that MMP-8 levels in UAP may reflect coronary plaque instability and that MMP-8 is a promising biomarker for UAP.