Resistance to antiandrogen therapy in patients with metastatic prostate cancer poses a major challenge, which, if overcome, may lead to significant advances in the treatment of these patients. Hormone resistance of prostate cancer develops, in part, from upregulation of antiapoptotic genes after androgen deprivation. Given the accumulating evidence that Survivin, a new member of the inhibitor of apoptosis (IAP) family, is associated with both cancer progression and drug resistance, we hypothesized that Survivin plays a potentially important role in hormone therapy resistance, and that targeting of Survivin may enhance sensitivity to antiandrogen therapy in prostate cancer. Patterns of Survivin expression were assessed in three prostate cancer cell lines LNCaP, PC-3, and DU-145 using quantitative Western analysis. All three cell lines were found to strongly express Survivin. In LNCaP cells with intact androgen receptors (ARs), it was observed that androgen stimulation with 5alpha-dihydrotestosterone (DHT) increased Survivin expression. Conversely, treatment with Flutamide decreased Survivin expression in LNCaP cells. We next studied the functional effect of Survivin on sensitivity to Flutamide. LNCaP cells were infected with replication-deficient adenoviruses encoding either wild-type Survivin pAd-S(WT) or a phosphorylation-defective Survivin Thr34 --> Ala dominant-negative mutant pAd-S(T34A), and then treated with Flutamide. Cell viability and apoptosis were assessed in vitro and in vivo. It was determined that Survivin can mediate resistance to such antiandrogen therapies based on our assays. Direct androgen stimulation resulted in pan-cell cycle expression of Survivin, which was found to be mediated by AKT, as it was determined that exogenous insulin-like growth factor-1 (IGF-1), a known activator of AKT signaling, could increase Survivin expression and result in pan-cell cycle expression even in AR-negative prostate cancer cell lines PC-3 and DU-145. Given this alternative mechanism of Survivin expression and our findings that Survivin can mediate resistance to Flutamide treatment, we further investigated whether IGF-1-mediated activation of Survivin via AKT could mediate resistance to antiandrogen therapy. Both in vitro and in vivo, this was found to be the case, supporting a novel mechanism of resistance to antiandrogen therapy. Our study indicates that upregulation of Survivin via IGF-1 signaling confers resistance to Flutamide in prostate cancer cells. Targeted inhibition of Survivin appears to enhance the therapeutic effects of Flutamide in vitro and in vivo, revealing a novel strategy to enhance sensitivity to androgen ablation therapy.