BACKGROUND: To compare intermittent treatment (IT) versus continuous treatment (CT) using cyproterone acetate (CPA) in bone metastatic prostate cancer patients, we conducted an open-label, multicenter randomized trial. Continuous androgen deprivation therapy is the standard treatment in metastatic prostate cancer. Intermittent treatment might maintain efficacy while toxicity and costs are reduced. METHODS: Patients received CPA 100 mg tid in the prephase. Patients with a PSA decline of ≥ 90 % or PSA <4 ng/ml were randomized. If patients were progressive, LHRH analogues were added. Primary end point was time to PSA progression. RESULTS: A total of 366 patients were recruited; 258 reached a good response after 3 or 6 months and were randomized. A total of 131 patients randomized to IT and 127 to CT. Patients on IT had an average of 1.7 episodes on CPA, before LHRH analogues were started. The mean time without treatment in IT was 463 days versus 422 days on treatment. There were statistical significant differences between IT and CT in 3 of the 5 functional scales of EORTC QLQ C 30; however, the clinical relevance of this finding appears modest. Symptom and potency scales showed significant advantages for IT. There were no differences in time to PSA progression on CPA, time to PSA and/or clinical progression on LHRH analogues and time to cancer-specific and overall survival. CONCLUSIONS: IT by CPA is associated with less symptoms and modest advantages in QOL domains. There were no differences in time to PSA progression, clinical progression or survival.