Objective: Osteoprotegerin (OPG), an inhibitor of osteoclastic bone resorption, has a variety of functions including anti-inflammatory effects and a possible cardiovascular protective role. Both low-grade chronic inflammation and cardiovascular risk are increased in women with the polycystic ovary syndrome (PCOS). We aimed to study serum OPG concentrations in PCOS patients. Design: Case-control study including 40 PCOS patients matched with 40 non-hyperandrogenic women for age and body mass index. Methods: Basal serum sampling and standard oral glucose tolerance test, and measurement of serum OPG concentrations by commercial ELISA. Results: Serum OPG concentrations were lower in women with PCOS compared with those of controls (304G120 vs 363G105 pg/ml respectively; FZ7.641, PZ0.007) independently of obesity. No differences were observed in serum receptor activator of nuclear factor-kB ligand (RANKL) levels and in the RANKL/OPG molar ratio. A multivariate linear regression model (R 2 Z0.208, FZ6.579, PZ0.001) showed that PCOS (bZK0.281, PZ0.008), obesity (bZK0.245, PZ0.022) and age (bZ0.296, PZ0.006) were predictive of serum OPG concentrations. Conclusions: Serum OPG concentrations are reduced in PCOS patients independently of obesity. Considering the anti-inflammatory effects of OPG, its reduced serum concentrations might contribute to the proinflammatory state and cardiovascular risk of PCOS patients.European Journal of Endocrinology 159 225-232