Regulation of osteoclastic activity is critical for understanding bone loss associated with the postmenopausal period. In vitro and animal studies have revealed the role of OPG as a decoy receptor that neutralizes the effect of RANKL on the differentiation and activation of osteoclasts. However, the role of the OPG-RANKL system in postmenopausal osteoporosis is controversial. Thus, the aim of this study was to investigate the relationship among circulating levels of OPG, RANKL, bone turnover markers (BTM), bone mineral density (BMD) and vertebral fractures in postmenopausal women. We determined anthropometric parameters, circulating OPG and RANKL, BTM, estradiol, BMD by dual X-ray absorptiometry at the lumbar spine (LS) and femoral neck (FN), and pre-existing vertebral fractures in 206 ambulatory postmenopausal women of a mean age of 62 years (SD 7). Circulating OPG was significantly related to age (r =0.158; P =0.023), years since menopause (r =0.167; P =0.016) and BMD (LS Z-score: r =0.240; P =0.001, FN Z-score: r =0.156; P =0.025). Over half of the women had undetectable RANKL (n =113; 54.9%). There were no significant differences in clinical variables, BTM or BMD among women with detectable vs. undetectable RANKL. OPG was found to be independently associated with osteoporosis (OR: 2.9, 1.4-5.9) and prevalent vertebral fractures (OR: 2.5, 1.2-5.4). We conclude that serum OPG levels are independently associated with bone mass and prevalent vertebral fractures in postmenopausal women.