We are proposing to conduct a non-comparative randomized phase II study of repetitive dosing of ipilimumab either alone or in combination with GM-CSF in patients with metastatic CRPC. The dosing interval for ipilimumab is based on the prior study which demonstrated drug levels ≥ 10 mg/mL (a minimum level required for CTLA-4 blockade in pre-clinical models) for greater than 28 days. Six doses of ipilimumab were chosen because six doses have been given safely in other trials. Maintenance dosing every three months is empirical, but this dosing frequency is based on discussions with Medarex, Inc. and Bristol-Meyers Squibb and is based on reports indicating the safety and potential efficacy of this maintenance regimen.