Objective: To investigate the role of osteoprotegerin (OPG) expression in occurrence and development of the abdominal aortic aneurysm (AAA). Methods: Twenty AAA tissue and 6 normal human abdominal aortic tissue samples were collected, and animal AAA model was created in rabbits using a patch technique (18 AAA rabbits were sacrificed to obtain tissue sample at 7, 21 and 35 d after model creation, respectively, with 6 rabbits undergoing sham operation as control). The OPG protein expressions in human AAA and normal human abdominal aortic tissues were detected by immunohistochemical staining, the protein and mRNA expressions of OPG and matrix metalloproteinase 9 (MMP-9) in the above human tissues and AAA tissues from animal models were measured by Western blot and RT-PCR method respectively, and the apoptosis of the vascular smooth muscle cells (VSMCs) in the tunica media of the AAA tissues from animal models was also determined by terminal transferase dUTP nick end labeling (TUNEL) assay. Results: The results of immunohistochemistry showed that OPG expression in human AAA tissue was increased compared with healthy abdominal aortic tissue, which became more evident with the increasing diameter of the aneurysm and was most evident in the ruptured AAA. Results of Western blot and RT-PCR demonstrated that the protein and mRNA expressions of both OPG and MMP-9 in the tissues from either the AAA biopsies or animal models were significantly higher than those in their controls (P<0.05), and the increases were progressive with the increasing diameter of the aneurysm and time duration after model creation. TUNEL assay showed that the apoptosis of VSMCs in model group was significantly increased versus control group with a growing trend towards prolonged period of model creation. Conclusion: OPG expression level is closely connected to the occurrence and development of AAA, and the mechanism may be associated with its stimulating MMP-9 activity and inducing apoptosis of VSMCs.