Roles of cyclin dependent kinase inhibitors, p21/Cip1 (p21) and p27/Kip1 (p27) in prostate cancer (PCa) progression is still not clear. Lower p27 protein expression in PCa tissues is often associated with poor prognosis, but prognostic significance of p21 is still controversial. Herein, we investigated the role of these molecules in determining PCa growth characteristics. We generated human PCa DU145 cell variants with knocked down levels of p21 (DU-p21) or p27 (DU-p27), or both (DUp21 + p27) via retroviral transduction of respective shRNAs and compared their various characteristics with empty vector-transduced DU145 (DU-EV) cells in vitro as well as in vivo. Knocking down either p21 or p27 did not show any significant change in doubling time, clonogenicity and cell cycle progression in DU145 cells, but simultaneous knock-down of both p21 and p27 significantly enhanced these parameters. In athymic mice, DU-p21 + p27 tumors showed higher growth rate than the comparable growth of DU-EV, DU-p21 and DU-p27 tumors. Concurrently, DU-p21 + p27 tumors had significantly higher proliferation rate, showing 54% and 48% increase in proliferating cell nuclear antigen (PCNA) and Ki-67-positive cells, respectively, compared to DU-EV tumors. DU-p21 + p27 tumors also showed higher microvessel density and increased expression of vascular endothelial growth factor (VEGF). Proliferation and angiogenic status of DU-p21 and DU-p27 tumors was comparable to DU-EV tumors. Both in vitro and in vivo results implicate that p21 and p27 have compensatory roles in advanced prostate cancer cells, and ablation or downmodulation of both these molecules essentially enhances the aggressive prostate carcinoma phenotype.