We analyzed 115 iliac crest bone marrow biopsy specimens from 101 patients with metastatic castration-resistant prostate cancer, divided into a test (n = 57) and a validation (n = 58) set. We developed a score based on computed tomography Hounsfield units and lactate dehydrogenase levels, which were associated with a positive biopsy result. The score can be used to select patients for whom a bone marrow biopsy will provide tissue for molecular characterization.