Background and aim: Osteoprotegerin(OPG) is a glycoprotein that acts by competing with receptor activator of nuclear factor-��B(RANK) for binding RANK ligand(RANKL), a critical cytokine for the differentiation, activation, and survival of osteoclasts. Recently, OPG has been reported to have correlation with the severity of coronary artery disease(CAD) and aortic calcification. The purpose of this study is to evaluate the usefulness of serum OPG measurement to predict coronary calcification suggesting complex atherosclerotic plaque in patients with CAD. Methods: Intravasular ultrasound(IVUS) examinations of the coronary arteries were perfomed in 65 subjects(mean age: 62짹10 yrs, M:F=46:19). The severity of coronary calcification was estimated by the largest arc of calcium within the most stenosed coronary segments measured with protractor and other routine IVUS parameters including plaque area and plaque burden were checked. Blood sample was collected at the time of coronary angiogram and serum OPG level was measured with ELISA. Results:1. Serum OPG levels showed good correlation with the severity of coronary calcification.(p=0.01, r=0.35). 2. Coronary calcifications tend to be positively correlated with age( p=0.06, r=0.21). 3. In patients with mild to moderate calcification less than 180^o of calcium deposition, serum OPG levels were correlated with coronary calcification independent of age(p=0.046 r=0.214). 4. Serum OPG levels were not significantly correlated with plaque area(p=0.445) and plaque burden(p=0.345), minimal luminal diameter(p=0.152), and minimal luminal area(P=0.183) of the lesions. Conclusion : In patients with CAD, serum OPG level correlates well with the severity of coronary calcification, especially in mild to moderate calcification. Therefore serum OPG can be a useful biochemical marker for coronary calcification and OPG itself might be involved in the process of coronary calcification.