This is a single-center, open-label (identity of assigned study drug will be known) study investigating the effect of abiraterone acetate on levels of testosterone and dihydrotestosterone (DHT) in bone marrow plasma of patients with metastatic CRPC and evaluating the difference in bone marrow androgen levels between patients with and without serum prostate specific antigen (PSA) decline. Approximately 60 medically or surgically castrated male patients with metastatic CRPC will be enrolled. The study will consist of screening, treatment, and follow-up periods. Patients will be treated orally (by mouth) with abiraterone acetate 1000 mg daily and prednisone 5 mg twice a day until clinical disease progression. Follow-up will continue until the patient dies, is lost to follow-up or withdraws informed consent. Bone marrow aspirates will be collected at Week 1 (predose), Week 9, and the final study visit. Safety will be monitored throughout the study.