Osteoprotegerin (OPG) is a secretory glycoprotein that belongs to the tumor necrosis factor receptor family. OPG-deficient mice develop severe osteoporosis and medial arterial calcification of the aorta and renal arteries. OPG immunoreactivity was demonstrated in the normal blood vessels and in early atherosclerotic lesions. A recent clinical study suggests that there is a significant correlation between elevated serum OPG levels and cardiovascular mortality. We examined whether serum OPG levels are associated with the progression of coronary artery disease (CAD).