PURPOSE: To our knowledge this study represents the first analysis monitoring the efficacy of cyproterone acetate (CPA) monotherapy for achieving castrate testosterone levels prior to administering a luteinizing hormone-releasing analogue (LHRHA) for treating prostate cancer in the prostate specific antigen (PSA) era. MATERIALS AND METHODS: Patients with untreated locally advanced or metastatic prostate cancer were recruited prospectively. Treatment involved a 28-day course of oral cyproterone acetate and LHRHA depot injection on day 14. Patients had serum PSA, luteinizing hormone and testosterone monitored at intervals during a 56-day period. RESULTS: A total of 15 patients with a mean age of 74 years completed the study. Near castrate serum testosterone was achieved on day 7 (mean +/- 95% CI 83.38 +/- 17.87 ng/dl). There was a significant testosterone increase after LHRHA administration on day 14 compared with the level of 160.23 +/- 36.60 ng/dl on day 16 (p <0.01). Serum luteinizing hormone mirrored testosterone, increasing from a mean of 4.93 +/- 0.61 to 15.4 +/- 6.12 nmol/l after LHRHA administration (p <0.01). Mean serum PSA demonstrated a decrease from 199.25 +/- 6.12 microg/l at day 0 to 43.77 +/- 33.08 microg/l by day 56. There was no increase in serum PSA after LHRHA administration. CONCLUSIONS: Two weeks of priming with CPA does not eliminate the surge in serum testosterone (testosterone flare) upon LHRHA administration but the testosterone increase does not exceed pretreatment levels. Furthermore, 2 weeks of CPA may not offer a benefit over 1 week in lowering serum testosterone. Finally, there is no increase in serum PSA when LHRHA is administered after priming with CPA.