This is a pilot study of mCRPC patients planning to undergo therapy with sipuleucel-T immunotherapy. Consenting patients will be randomized 3:1 between immediate sipuleucel-T immunotherapy followed by lymph node biopsy (the post-treatment experimental group) or immediate lymph node biopsy followed by sipuleucel-T immunotherapy (the pre-treatment control group). Peripheral blood will be collected before, during, and after treatment with sipuleucel-T and evaluated for evidence of sipuleucel-T induced immune activation. Lymph nodes collected at biopsy will also be evaluated for evidence of sipuleucel-T induced immune activation. Patients will be followed for 3 months for safety and 6 months for disease progression.