Background. Osteoprotegerin (OPG) is a glycoprotein that inhibits osteoclast differentiation and activity. OPG-deficient mice develop severe osteoporosis and medial arterial calcification. The expression of OPG is detected in early atherosclerotic lesions in non-uraemic patients. We examined whether serum OPG is associated with aortic calcification in haemodialysis patients. Methods. Serum OPG was measured in 102 patients who were undergoing haemodialysis. The aortic calcification index (ACI) was assessed by computed tomography scans. Results. Serum OPG level, measured by enzyme-linked immunosorbent assay, was significantly greater in patients with higher ACI than in those with lower ACI. There was a direct relationship between ACI and serum OPG levels and a positive association between OPG and ACI (r = 0.483, P<0.0001). Multiple regression analyses indicated that serum OPG levels were independently associated with the severity of aortic calcification (P<0.0001). Conclusions. These findings show that serum OPG levels are associated with the extent of vascular calcification, suggesting that OPG may be involved in the development of vascular calcification in haemodialysis patients.