Ipilimumab (Yervoy®; Bristol-Myers Squibb, NY, USA) is a fully human monoclonal antibody targeting CTLA-4 and is approved for the treatment of metastatic melanoma. Preclinical and clinical studies have shown its activity in a number of different cancer types, including prostate cancer. Recently, the results from a Phase III study of ipilimumab in prostate cancer patients with prior docetaxel therapy were reported. Although the study did not meet the primary end point of improved overall survival, prespecified subset analyses suggested that ipilimumab may be more active in men with lower disease burden, which suggests that immunotherapy should be tested early in men with castration-refractory prostate cancer. Immune-related adverse events are common and most can be well managed with standard immunosuppressive algorithms.