Abstract The treatment scope for patients with metastatic castrate-resistant prostate cancer (mCRPC) is rapidly expanding. On May 15, 2013, the U.S. Food and Drug Administration (FDA) approved radium-223 chloride (223RaCl2) for the treatment of mCRPC patients whose metastases are limited to the bones. Radium-223 is an α-emitting alkaline earth metal ion, which, similar to calcium ions, accumulates in the bone. In a phase III study (ALSYMPCA), mCRPC patients with bone metastases received best standard-of-care treatment with placebo or 223RaCl2. At a prespecified interim analysis, the primary endpoint of median overall survival was significantly extended by 3.6 months in patients treated with radium-223 compared with placebo (P < 0.001). The radioisotope was well tolerated and gave limited bone marrow suppression. 223RaCl2 is the first bone-targeting antitumor therapy that received FDA approval based on a significant extended median overall survival. Further studies are required to optimize its dosing and to confirm its efficacy and safety in cancer patients. Clin Cancer Res; 19(21); 5822–7. ©2013 AACR.