The relationship between sex steroids and osteoprotegerin (OPG) in patients with prostate cancer is not well established. Our aim was to evaluate serum OPG levels in patients with prostate cancer and its relationship with sex steroids, bone mineral density, bone turnover markers, and fractures. We performed a cross-sectional study including 91 patients with prostate cancer. We determined: bone mineral density by dual-energy x-ray absorptiometry, bone turnover markers, serum levels of sex steroids and osteoprotegerin, and prevalent radiographic vertebral fractures. Serum OPG levels were higher in patients with vertebral fractures (8.02 ± 2.0 vs 4.91 ± 0.28 pmol/L; P < .05). OPG level and the duration of hormonal therapy were related (r = 0.299, P = .004), but this association did not persist after adjustment for age. In patients without androgen deprivation therapy, serum OPG levels were correlated with the levels of total testosterone (r = 0.508, P = .001) and bioavailable testosterone (r = 0.311, P = .037). In patients receiving androgen deprivation therapy, serum OPG levels were correlated with levels of total estradiol (r = 0.199, P = .18), bioavailable estradiol (r = 0.37, P = .009), and free estradiol (r = 0.349, P = .016). In conclusion, in patients with prostate cancer treated with androgen deprivation therapy, serum OPG levels were correlated with the levels of total estradiol, bioavailable estradiol, and free estradiol. Our hypothesis is that in patients with androgen deprivation therapy, the higher relative estrogen levels could stimulate OPG production in response to the higher resorption state. Future prospective studies are needed to clarify the role of OPG in androgen deprivation therapy-mediated bone loss.