We quantified T1, T2, and proton density (PD) of bone metastases in castration-resistant prostate cancer with synthetic MRI (SyMRI). Bone foci of interest were classified into four groups; active disease, red bone marrow (RBM), inactive disease without calcification (Inactive C(-)), and inactive disease with calcification (Inactive C(+)). Active disease group showed very high PD, and Inactive C(+) group showed very low PD. Both Inactive C(-) and RBM showed medium values. Significant differences were noted among these three divisions. SyMRI thus shows clinical potential to differentiate active/inactive lesions, calcifications, and red bone marrow in castration-resistant prostate cancer.