This study aimed to analyze the relationship between the levels of two tumor markers—carcinoembryonic antigen (CEA) and Ca19-9 antigen—measured in serum (sCEA and sCa19-9) and intraoperative peritoneal lavage fluid (pCEA and pCa19-9) and the stage of gastric cancer (including the presence of cancer cells in cytology from abdominal cavity fluid). The gene discussed is CEACAM5; the disease is neoplasm.