Prostate cancer is the most common cancer in men in America. Currently, steroid receptor coactivators have been proposed to mediate the development and progression of prostate cancer, at times in a steroid-independent manner. Steroid receptor coactivator-3 (SRC-3, p/CIP, AIB1, ACTR, RAC3, and TRAM-1) is a member of the p160 family of coactivators for nuclear hormone receptors including the androgen receptor. SRC-3 is frequently amplified or overexpressed in a number of cancers. However, the role of SRC-3 in cancer cell proliferation and survival is still poorly understood. In this study, we show that SRC-3 is overexpressed in prostate cancer patients and its overexpression correlates with prostate cancer proliferation and is inversely correlated with apoptosis. Consistent with patient data, we have observed that reduction of SRC-3 expression by small interfering RNA decreases proliferation, delays the G₁-S transition, and increases cell apoptosis of different prostate cancer cell lines. Furthermore, with decreased SRC-3 expression, proliferating cell nuclear antigen and Bcl-2 expression, as well as bromodeoxyuridine incorporation in prostate cancer cells are reduced. Finally, knockdown of SRC-3 with inducible short hairpin RNA expression in prostate cancer cells decreased tumor growth in nude mice. Taken together, these findings indicate that SRC-3 is an important regulator of prostate cancer proliferation and survival.