T he AKT/mammalian target of rapamycin (AKT/mTOR) and ERK MAPK signaling pathways have been shown to cooperate in prostate cancer progression and the transition to androgen-independent disease. We have now tested the effects of combinatorial inhibition of these pathways on prostate tumorigenicity by performing preclinical studies using a genetically engineered mouse model of prostate cancer. We report here that combination therapy using rapamycin, an inhibitor of mTOR, and PD0325901, an inhibitor of MAPK kinase 1 (MEK; the kinase directly upstream of ERK), inhibited cell growth in cultured prostate cancer cell lines and tumor growth particularly for androgen-independent prostate tumors in the mouse model. We further showed that such inhibition leads to inhibition of proliferation and upregulated expression of the apoptotic regulator Bcl-2–interacting mediator of cell death (Bim). Furthermore, analyses of human prostate cancer tissue microarrays demonstrated that AKT/mTOR and ERK MAPK signaling pathways are often coordinately deregulated during prostate cancer progression in humans. We therefore propose that combination therapy targeting AKT/mTOR and ERK MAPK signaling pathways may be an effective treatment for patients with advanced prostate cancer, in particular those with hormone-refractory disease.