BACKGROUND. Androgens play a crucial role in prostate cancer, hence the androgenic pathway has become an important target of therapeutic intervention. Previously we discovered that gene fusions between the 5'-untranslated region of androgen regulated gene TMPRSS2 and the ETS transcription factor family members were present in a majority of the prostate cancer cases. The resulting aberrant overexpression of ETS genes drives tumor progression. METHODS. Here, we evaluated the expression levels of 5α-reductase isoenzymes in prostate cancer cell lines and tissues. We tested the effect of dutasteride, a 5α-reductase inhibitor, in TMPRSS2-ERG fusion-positive VCaP cell proliferation and cell invasion. We also evaluated the effect of dutasteride on the TMPRSS2-ERG fusion gene expression. Finally, we tested dutasteride alone or in combination with an anti-androgen in VCaP cell xenografts tumor model. RESULTS. Our data showed that 5α-reductase SRD5A1 and SRD5A3 isoenzymes that are responsible for the conversion of testosterone to DHT, are highly expressed in metastatic prostate cancer compared to benign and localized prostate cancer. Dutasteride treatment attenuated VCaP cell proliferation and invasion. VCaP cells pre-treated with dutasteride showed a reduction in ERG and PSA expression. In vivo studies demonstrated that dutasteride in combination with the anti-androgen bicalutamide significantly decreased tumor burden in VCaP cell xenograft model. CONCLUSIONS. Our findings suggest that dutasteride can inhibit ERG fusion-positive cell growth and in combination with anti-androgen, significantly reduce the tumor burden. Our study suggests that anti-androgens used in combination with dutasteride could synergistically augment the therapeutic efficacy in the treatment of ETS-positive prostate cancer. Copyright © 2012 Wiley Periodicals, Inc.