Background. Most acute coronary syndromes are caused by the fracture of a vulnerable atherosclerotic plaque. These plaques are thin cap fibroatheromas, which can only be detected with invasive coronary imaging techniques. It is necessary to find a non-invasive biomarker of these vulnerable plaques in order to identify patients at risk without a coronary angiography. Metalloproteinase-1 is an enzyme involved in extracellular matrix metabolism which has been correlated with the rupture of atherosclerotic plaques. Its serum levels in patients with vulnerable plaques remain unknown. Methods. Patients with suspected stable coronary artery disease undergoing coronary angiography in our hospital were included. The coronary arteries were studied with optical coherence tomography to detect vulnerable plaques. Blood samples were taken from a peripheral vein and from the coronary sinus, to assess metalloproteinase-1 levels. Results. Fifty-one patients were included, 13 of whom had at least one vulnerable plaque. There were not significant differences in clinical characteristics, lipid profile or C reactive protein levels, between patients with or without vulnerable plaques. Patients with vulnerable plaques had significant higher metalloproteinase-1 levels both in peripheral (7330 5541 vs 2894 1783 pg/ml, p=0.025) and coronary sinus serum (6012 3854 vs 2707 1252 pg/ml, p=0.047). Conclusions. Patients with vulnerable plaques had significantly higher metalloproteinase-1 serum levels. Further studies with clinical follow up are needed to assess the prognostic value of serum metalloproteinase-1.