Novel estrogen therapy has the potential to be efficacious, with a favorable adverse event profile, in castration-resistant prostate cancer (CRPC). We performed a phase 2 trial to assess the ability of GTx-758, an oral selective estrogen receptor alpha agonist, to result in a ≥ 50% PSA decline by day 90, modulate free testosterone and sex hormone-binding globulin (SHBG) levels, and affect estrogen deficiency adverse events.