We analyzed the peripheral blood of patients with gastrointestinal tract cancer at different stages to assess the presence of carcinoembryonic antigen (CEA) mRNA by reverse transcriptase-polymerase chain reaction (RT-PCR), which we used as an indicator for micrometastatic malignant cells. A total of 35 gastric, 24 colorectal, 4 esophageal and 4 biliary tract cancer patients and nine normal healthy subjects were studied. No CEA mRNA was detected in the nine normal healthy volunteers. CEA mRNA was detected in 100% (10/10) of metastatic, 33.3% (3/9) of early gastric cancer (EGC), and 18.8% (3/16) resectable gastric cancer patients, respectively. In colorectal cancer, 55.6% (5/9) of metastatic cancers were positive for CEA mRNA, and 26.7% (4/15) Duke stage B/C showed positive. One patient with stage III gastric cancer who was negative CEA mRNA initially and turned positive during follow-up, developed multiple bone metastasis one month later. Another stage III patient, who was positive for CEA mRNA, preoperatively revealed early relapse in two months. These results suggest that the identification of circulating tumor cells using RT-PCR for the detection of CEA mRNA is feasible and this analysis may be a promising tool for early detection of micrometastatic circulating malignant cells in patients with gastrointestinal tract cancer.