External beam radiotherapy (RT) is often used in an attempt to cure localized prostate cancer (PCa), but it is only palliative against disseminated disease. Raf kinase inhibitory protein (RKIP) is a metastasis suppressor whose expression is reduced in approximately 50% of localized PCa tissues and is absent in metastases. Chemotherapeutic agents have been shown to induce tumor apoptosis through induction of RKIP expression. Our goal was to test whether RT similarly induces apoptosis through induction of RKIP expression.The C4-2B PCa cell line was engineered to overexpress or underexpress RKIP. The engineered cells were tested for apoptosis in cell culture and tumor regression in mice after RT.RT induced both RKIP expression and apoptosis of PCa cells. Overexpression of RKIP sensitized PCa cells to radiation-induced apoptosis. In contrast, short-hairpin targeting of RKIP, so that RT could not induce RKIP expression, protected cells from radiation-induced apoptosis. In a murine model, knockdown of RKIP in PCa cells diminished radiation-induced apoptosis. Molecular concept mapping of genes altered on manipulation of RKIP expression revealed an inverse correlation with the concept of genes altered by RT.The data presented in this report indicate that the loss of RKIP, as seen in primary PCa tumors and metastases, confers protection against radiation-induced apoptosis. Therefore, it is conceivable that the loss of RKIP confers a growth advantage on PCa cells at distant sites, because the loss of RKIP would decrease apoptosis, favoring proliferation.