For the early detection of tumor-related aberrant DNA in the serum of patients, we examined promoter hypermethylation of the p16 gene using methylation-specific PCR (MSP) in paired tumor and serum samples of 60 gastric cancer patients. Aberrant p16 methylation was found in 23 of 60 (38%) primary gastric cancers, but in none of the corresponding gastric mucosae. Of these 23 patients, 6 (26%) exhibited the same alteration in their serum DNA. As a control, we screened for aberrant methylation in the serum DNA of 37 patients with gastric cancers whose corresponding tumor DNA had no methylation in the p16 promoter. We also screened for methylation in the serum DNA of 16 non-cancer individuals. No methylation was found in serum DNA of these control groups. Our results suggest that p16 methylation would be a good marker for the detection of tumor DNA in the serum of primary gastric cancer patients.