Background: This multicentre, randomised, open label, phase II/III study aimed to investigate the potential benefit of adding risedronate (R) to docetaxel (D) in patients with metastatic Castration Resistant Prostate Cancer (CRPC). Patients and methods: CRPC patients with bone metastasis were randomly assigned to receive D 75 mg/m(2) every 3 weeks and prednisone as first line chemotherapy, with or without R 30 mg oral once daily. The primary end-point was time to progression (TTP). A composite end-point of objective progression by RECIST criteria, PSA progression, or pain progression, whichever occurred first, was applied. The study had 80% power to detect an improvement of 30% in median TTP in the DR group (two-sided alph Results: Five hundred and ninety-two men (301 D versus 291 DR) were randomised. TTP was 7.4 [D] versus 6.5 [DR] months (p = 0.75). PSA and pain response rates were similar, 66.3% [D] versus 65.9% [DR] and 27.9% [D] versus 31.2% [DR], respectively. Median overall survival (OS) was 18.4 [D] versus 19.2 [DR] months (p = 0.33). There were no differences in toxicity. Conclusion: The addition of the third generation bisphosphonate, risedronate, in the setting of effective first line docetaxel based chemotherapy did not increase efficacy, as indicated by the lack of improvement in TTP, OS, PSA- and pain response. (C) 2012 Elsevier Ltd. All rights reserved.