Osteoprotegerin (OPG) is membrane bound in the immune system but can also be produced and secreted almost everywhere in the organism, so that it is mainly available in soluble form. So far, the OPG/RANKL/RANK system has been most intensively studied in bone. The binding of RANKL on its receptor RANK, which is expressed by osteoclasts, activates a number of osteoclastic cell functions. OPG also has a key function in the vascular system. Patients with coronary heart disease (CAD) have elevated OPG serum levels, probably as a sign of ischemic or inflammatory endothelial damage. Elevated OPG levels were also found in patients with advanced heart failure, whereby OPG correlated with pro BNP (brain natriuretic peptide) and was a predictor for cardiovascular morbidity and mortality.