Aim: To investigate the clinical and pathological relevance of detection of circulating tumor cells (CTC) in the peripheral blood of gastric carcinoma patients before operation. Patients and Methods: Fifty patients with gastric adenocarcinoma were analysed prospectively. Patients were divided into two groups according to the extent of the tumor. Group I (unresectable) consisted of 22, and group II (resectable) consisted of 28 patients. Peripheral blood samples were collected pre-operatively from all 50 patients as well as from ten healthy controls and analyzed for carcinoembryonic antigen (CEA) and cytokeratin-19 (CK-19) messenger ribonucleic acids (mRNAs). Tumor localisation, stage, presence of signet cell formation, nodal metastases, serousal and lymphovascular invasion were recorded for all patients. Results: Expression of CK-19 was detected in 24 (48%), and CEA in 10 (20%) cases. Nine patients (40%) in group I and 15 (53.6%) in group II were positive for CK-19 expression. CEA expression was more frequent among group I patients (6 vs. 4 cases). There was no significant difference between the groups in the expression of CK-19 and CEA mRNA, tumor localisation, presence of signet formation, and presence and extent of nodal metastases. Patients with major vascular invasion (MVI) expressed significantly higher levels of CTC mRNA compared to those without MVI (p = 0.023 for CEA, and p = 0.009 for CK-19). The median 1 and 2-year survival was 9.5 and 10.5 months for group I, and 20 and 28.5 months for group II, respectively (p = 0.001). The mean survival was 6.7 months for patients with MVI, and 30.2 months for those without MVI (p = 0.0001). Conclusions: High levels of CTCs were observed in patients with MVI invasion, rather than other causes of unresectability. It can be suggested that expression of both CEA and CK-19 in the peripheral blood of gastric cancer patients are strong predictors of MVI and significantly worse survival rates. Copyright © Experimental Oncology, 2010.