Methylation of the glutathione S-transferase P1 (GSTP1) gene has been described as a highly specific and sensitive biomarker for prostate cancer. However, at present, it is not known whether methylation represses GSTP1 gene expression in human prostate cancer. We found the GSTP1 gene promoter to be completely methylated in the LNCaP prostate cancer cell line, where this gene is transcriptionally inactive. In contrast, Du145 and PC3 prostate cancer cells express the GSTP1 gene and exhibit methylated and unmethylated GSTP1 alleles. In a transient transfection assay using LNCaP cells, methylation of the GSTP1 promoter-driven luciferase reporter vector (GSTP1-pGL3) resulted in a >20-fold inhibition of transcription, and this repression was not relieved by the presence of a histone deacetylase inhibitor, trichostatin A (TSA). Treatment of LNCaP cells with a DNA methyltransferase inhibitor, 5-Aza-2'-deoxycytidine, resulted in demethylation and activation of the GSTP1 gene. In contrast, TSA treatment failed to demethylate or activate the GSTP1 gene. Fully methylated but not unmethylated GSTP1 promoter fragment was shown to bind to a complex similar to methyl cytosine-binding protein complex 1 that contains methyl-CpG-binding domain 2 protein (MBD2) in electrophoretic mobility shift assays using LNCaP cell nuclear extracts. These data demonstrate that cytosine methylation can repress GSTP1 gene expression in LNCaP prostate cancer cells and that this effect is possibly mediated by a methyl cytosine-binding protein complex 1-like complex. Furthermore, these data also support the notion of the dominance of methylation over TSA-sensitive histone deacetylation in silencing genes with a high CpG density in the promoter region.