This is an open-label randomized phase II study. Patients are randomized so as to achieve uniform patient cohorts treated on each regimen. Twenty-seven patients will be required per treatment arm, and a total of 54 prostate cancer patients will be required to complete this study. The study will assess for clinical activity by Prostate Specific Antigen (PSA) response, of both single agent ipilimumab and the combination of GM-CSF and ipilimumab in chemotherapy-naïve patients with metastatic castrate resistant prostate cancer.