Urokinase plasminogen activator (uPA) promotes tumor invasion and metastasis in several malignancies including prostate cancer, one of the most commonly detected male cancers that result in a high incidence of mortality. In the present study we have examined the differential regulation of uPA gene expression in different stages of prostate cancer by DNA methylation. We determined levels of uPA expression in normal prostate epithelial cells (PrEC) and in hormone-responsive (LNCaP) and -insensitive (PC-3) prostate cancer cell lines. We found that uPA is expressed only in the highly invasive PC-3 cells where the uPA promoter is unmethylated. The lack of uPA expression in PrEC and LNCaP cells, where uPA promoter is highly methylated, is due to suppression of uPA gene transcription by DNA methylation. Treatment of LNCaP cells with 5'-azacytidine, a potent demethylating agent, resulted in induction of uPA mRNA expression, uPA activity, and higher invasive capacity in vitro. Additionally, a marked increase in tumor volume was observed after inoculation of these cells into the flank of male BALB/c (nu/nu) mice. Collectively these studies have demonstrated that DNA methylation is the underlying molecular mechanism responsible for uPA gene silencing in normal and early stages of prostate cancer, which has a direct effect on tumor cell invasion and growth in vitro and in vivo.