PURPOSE We contrasted the endocrinological and biochemical efficacies of abarelix depot, a pure gonadotropin-releasing hormone antagonist, with a prospective concurrent control cohort receiving luteinizing hormone releasing hormone (LH-RH) agonists with or without antiandrogen for treatment of patients with prostate cancer receiving initial hormonal therapy. MATERIALS AND METHODS In this phase 2 open label study 242 patients with prostate cancer requiring initial hormonal treatment received abarelix depot (209) or LH-RH agonists (33) with or without antiandrogen. A total of 100 mg. abarelix depot was delivered intramuscularly every 28 days with an additional injection on day 15. LH-RH agonists with or without antiandrogen were administered according to the depot formulation used. Endocrine efficacy was measured by the absence of testosterone surge and rapidity of castration onset. The rate of prostate specific antigen decrease was assessed. RESULTS No patient treated with abarelix depot had testosterone surge during week 1 compared with 82% of those treated with LH-RH agonists. The concomitant administration of antiandrogen had no effect. During the first week of drug administration, in 75% of patients treated with abarelix depot and in 0% of those treated with LH-RH agonist medical castration was achieved. Prostate specific antigen decrease was faster, with no flare or surge in patients treated with abarelix depot. Abarelix depot was well tolerated. CONCLUSIONS Abarelix depot represents a new class of hormonal therapy, gonadotropin releasing hormone antagonists, that has rapid medical castration and avoids the testosterone surge characteristic of LH-RH agonists.