Metastatic renal cell carcinoma is resistant to current therapies. The phosphoinositide 3-kinase (PI3K)/Akt signaling cascade induces cell growth, cell transformation, and neovascularization. We evaluated whether targeting this pathway could be of therapeutic value against human renal cell carcinoma. The activation of the PI3K/Akt pathway and its role in renal cell carcinoma progression was evaluated in vitro in seven human cell lines by Western blot, cell counting, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide, terminal deoxyribonucleotide transferase–mediated nick-end labeling assays, and fluorescence-activated cell sorting analysis, using two PI3K inhibitors, LY294002 and wortmannin, as well as by transfection with various Akt constructs and through Akt knockdown by small interfering RNA (siRNA). In vivo nude mice bearing human renal cell carcinoma tumor xenografts were treated with LY294002 (75 mg/kg/wk, 4 weeks, i.p.). Tumor growth was measured and tumors were subjected to Western blot and immunohistochemical analysis. Akt was constitutively activated in all cell lines. Constitutive phosphorylation of glycogen synthase kinase-3 (GSK-3) was observed in all cell lines, whereas forkhead transcription factor and mammalian target of rapamycin, although expressed, were not constitutively phosphorylated. Exposure to LY294002 or wortmannin decreased Akt activation and GSK-3 phosphorylation and reduced cell growth by up to 70% through induction of cell apoptosis. These effects were confirmed by transfection experiments with Akt constructs or Akt siRNA. Importantly, LY294002 induced up to 50% tumor regression in mice through tumor cell apoptosis. Tumor neovascularization was significantly increased by LY294002 treatment. Blood chemistries showed no adverse effects of the treatment. Our results suggest an important role of PI3K/Akt inhibitors as a potentially useful treatment for patients with renal cell carcinoma. (Cancer Res 2006; 66(10): 5130-42)