This study will assess different doses and regimens of radium-223 dichloride on the incidence of symptomatic skeletal events. Eligible subjects must have castration resistant prostate cancer with 2 or more skeletal metastases documented within 8 weeks of randomization. Subjects will be randomized to one of 3 treatment arms in a 1:1:1 fashion: a standard regimen of radium-223 dichloride of 50 kBq/kg (55 kBq/kg after implementation of NIST update) injections every month for 6 months, a high dose regimen of 80 kBq/kg (88 kBq/kg after implementation of NIST update)injections every month for 6 months or an extended duration regimen of 50 kBq/kg (55 kBq/kg after implementation of NIST update) injections every month for 12 months. Following the treatment phase, subjects will be followed up every 12 weeks for a minimum of 2 years, at which point they will enter a long term follow-up period during which they are seen every 6 months for up to 7 years after the last dose of radium dichloride. Symptomatic skeletal event and safety endpoints will be assessed at each clinic visit. Pain and analgesic use data will be collected every 4 weeks through Week 48. Additionally, radiological assessments including MRI/CT of the abdomen and pelvis and chest CT, as well as technetium-99 bone scans will be performed at Weeks 8, 16, and 24 and continue every 12 weeks thereafter until disease progression is documented in either the bone or in soft tissue. Radiological imaging will be evaluated by blinded central review.