Many genetic and environmental factors contribute to development of cancer, but DNA methylation may provide a link between these influences. Genome stability and normal gene expression are largely maintained by a fixed and predetermined pattern of DNA methylation. In cancer, this idealistic scenario is disrupted by an interesting phenomenon: the hypermethylation of regulatory regions called CpG islands in some tumour suppressor genes--eg, BRCA1, hMLH1, p16INK4a, APC, VHL--which causes their inactivation. Development of new techniques that couple bisulphite modification with PCR has enabled these alterations to be studied in all types of biological fluids and archived tissues. Potentially, there are four types of translational studies that can be used to investigate the aberrant pattern of DNA methylation in cancer. First, CpG island hypermethylation can be used as a marker to identify cancer cells from biological samples, eg, serum and urine. This technique is highly sensitive and informative because profiles of tumour-suppressor-gene inactivation are specific to particular cancers. Second, single and combined genes that are inactivated by promoter hypermethylation, such as p16INK4a and DAPK, can be used as prognostic factors. Third, products of genes that are silenced by DNA methylation can be used as biomarkers of response to chemotherapy or hormone therapy--eg, the DNA repair O6-methylguanine-DNA methyltransferase and the oestrogen receptor. Finally, dormant tumour suppressor genes can be reactivated by DNA demethylating drugs, with the aim of reversing the neoplastic phenotype. These are new avenues worth exploring in the fight against cancer.