Understanding what triggers hypermethylation of tumour suppressor genes in cancer cells is critical if we are to discern the role of methylation in the oncogenic process. CpG sites in CpG island promoters, that span most tumour suppressor genes, remain unmethylated in the normal cell, despite the fact that CpG sites are the prime target for de novo methylation by the DNA methyltransferases. The CpG island-associated with the GSTP1 gene is an intriguing example of a CpG rich region which is susceptible to hypermethylation in the majority of prostate tumours and yet is unmethylated in the normal prostate cell. In this study we evaluate a number of factors purported to be involved in hypermethylation to test their role in triggering hypermethylation of GSTP1 in prostate cancer DU145 and LNCaP cells. We find that hypermethylation is not associated with (1) elevated expression of the DNA methyltranferases, or (2) removal of Sp1 transcription factor binding sites in the CpG island or (3) removal of CpG island boundary elements or (4) prior gene silencing. Instead our results support a model that requires a combination of prior gene silencing and random "seeds" of methylation to trigger hypermethylation of the GSTP1 gene in the prostate cancer cell. We propose that the GSTP1 gene is initially silenced in the prostate cancer and random sites of methylation accumulate that result in subsequent hypermethylation and chromatin remodelling.