STAMP1 is predicted to encode a six-transmembrane protein whose expression is highly prostate enriched and is deregulated in prostate cancer. However, the biological role of STAMP1 in prostate cancer cells, or its expression profile at the protein level, is unknown. Here, we find that ectopic expression of STAMP1 significantly increased proliferation of DU145 prostate cancer cells as well as COS-7 cells in vitro; conversely, small interfering RNA–mediated knockdown of STAMP1 expression in LNCaP cells inhibited cell growth and, at least partially, induced cell cycle arrest. In parallel, there were alterations in cell cycle–regulatory gene expression. Knockdown of STAMP1 expression in LNCaP cells also induced significant apoptosis under basal conditions as well as in response to tumor necrosis factor–related apoptosis-inducing ligand (TRAIL) alone, or TRAIL + AKT inhibitor LY294002, previously established apoptotic agents in LNCaP cells. Consistently, LNCaP cells with short hairpin RNA–mediated knockdown of STAMP1 were dramatically retarded in their ability to grow as xenografts in nude mice. Interestingly, activation of extracellular signal-regulated kinase, which has previously been implicated in prostate cancer progression, was significantly increased on ectopic expression of STAMP1 in DU145 cells and, conversely, was strongly downregulated on STAMP1 knockdown in LNCaP cells. In the normal prostate, STAMP1 protein is localized to the cytosol and the cell membrane of the prostate epithelial cells; furthermore, its expression is increased in prostate cancer compared with normal prostate. Taken together, these data suggest that STAMP1 is required for prostate cancer growth, which may be a useful target in prostate cancer treatment. Cancer Res; 70(14); 5818–28. ©2010 AACR.