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AIM: To evaluate the diagnostic role of serum RASSF1A promoter hypermethylation in gastric and colorectal adenocarcinoma.METHODS: Methylation-specific polymerase chain reaction (MSPCR) was used to examine the promoter methylation status of the serum RASSF1A gene in 47 gastric adenocarcinoma patients, 45 colorectal adenocarcinoma patients, 60 patients with benign gastrointestinal disease (30 with benign gastric disease and 30 with benign colorectal disease), and 30 healthy donor controls. A paired study of RASSF1A promoter methylation status in primary tumor, adjacent normal tissue, and postoperative serum were conducted in 25 gastric and colorectal adenocarcinoma patients who later were underwent surgical therapy.RESULTS: The frequencies of detection of serum RASSF1A promoter hypermethylation in gastric (34.0%)and colorectal (28.9%) adenocarcinoma patients were significantly higher than those in patients with benign gastric (3.3%) or colorectal (6.7%) disease or in healthy donors (0%) (P ＜ 0.01). The methylation status of RASSF1A promoter in serum samples was consistent with that in paired primary tumors, and the MSPCR results for RASSF1A promoter methylation status in paired preoperative samples were consistent with those in postoperative serum samples. The serum RASSF1A promoter hypermethylation did not correlate with patient sex, age, tumor differentiation grade, surgical therapy,or serum carcinoembryonic antigen level. Although the serum RASSF1A promoter hypermethylation frequency tended to be higher in patients with distant metastases,there was no correlation between methylation status and metastasis.CONCLUSION: Aberrant CpG island methylation within the promoter region of RASSF1A is a promising biomarker for gastric and colorectal cancer.