The concentration of cell-free DNA and promoter methylation status of the MGMT, p15, and hMLH1 genes were analyzed by a fluorescence-based assay and methylation-specific PCR (MSP) in the blood of gastric cancer patients (n= 20) and healthy subjects (n= 22). Gastric cancer patients were characterized by an increased concentration of circulating DNA in the plasma; the amount of cell-surface-bound DNA was not decreased compared with controls and amounted to 80 +/- 15% of the total circulating DNA. MSP analysis of three genes in the cell-surface-bound DNA permits the detection of gastric cancer patients with a sensitivity of 75% and a specificity of 54%. Thus, the cell-surface-bound DNA is a convenient source of DNA for MSP analysis of cancer-specific markers. The data on the presence of methylated DNA in plasma combined with the analysis of other cancer-related changes in DNA can significantly contribute to cancer diagnostics.