Prostate cancer cells require high rates of de novo fatty acid synthesis and protein synthesis for their rapid growth. We report here that the growth of these cells is markedly diminished by incubation with activators of AMP-activated protein kinase (AMPK), a fuel-sensing enzyme that has been shown to diminish both of these processes in intact tissues. Inhibition of cell growth was observed when AMPK was activated by either 5-aminoimidazole-4-carboxamide riboside (AICAR) or the thiazolidinedione rosiglitazone. Thus, a 90% inhibition of the growth of androgen-independent (DU145, PC3) and androgen-sensitive (LNCaP) cells was achieved after 4 days of exposure to one or both of these agents. Where studied, this was associated with a decrease in the concentration of malonyl CoA, an intermediate of de novo fatty acid synthesis, and an increase in expression of the cell cycle inhibitor p21. In addition, AICAR inhibited two key enzymes involved in protein synthesis, mTOR and p70S6K, and blocked the ability of the androgen R1881 to increase cell growth and the expression of two enzymes for de novo fatty acid synthesis, acetyl CoA carboxylase and fatty acid synthase, in the LNCaP cells. The results suggest that AMPK is a potential target for the treatment of prostate cancer.