In this study, we examined several molecular markers in prostate and breast cancer patients and in normal individuals. The markers tested were: variations in the quantity of plasma DNA, glutathione-S-transferase P1 gene (GSTP1), Ras association domain family 1A (RASSF1A), and ataxia telangiectasia mutated (ATM) methylation status in plasma, carcinoembryonic antigen (CEA) and prostate-specific membrane antigen (PSMA) mRNA in peripheral blood mononuclear cells (PBMC) and plasma samples from prostate cancer patients. DNA quantification in plasma was performed using real-time PCR (RT-PCR). We assessed the methylation status of GSTP1 in plasma DNA using methylation-specific PCR (MSP) assay, while the methylation status of RASSF1A and ATM genes was examined by the MethyLight technology. RT-PCR analysis was used for the detection of mRNA, PSMA, and CEA. In 58.3% of newly diagnosed prostate cancer patients and 26.7% of prostate cancer patients under therapy, plasma DNA levels were increased. Additionally, 48.5% of breast cancer patients showed plasma DNA levels above the cutoff limit. GSTP1 Promotor hypermethylation was detectable in 75% of plasma samples obtained from patients with newly diagnosed prostate cancer and in 36.8% of patients under therapy, whereas 26% and 14% of the breast cancer patients tested were positive for RASSF1A and ATM methylation, respectively. The combination of DNA load and promotor methylation status identified 88% of prostate cancer patients and 54% of breast cancer patients. This study shows that free-circulating DNA can be detected in cancer patients compared with disease-free individuals, and suggests a new, noninvasive approach for early detection of cancer.