The detection of isolated (circulating or disseminated) tumour cells (ITC) in patients with cancer requires very sensitive methods, as such cells are very rare. In the present study, the method that combines the negative isolation of CD45- leukocytes from the blood and bone marrow of patients with gastric cancer by flow cytometry, followed by the positive isolation of single cytokeratin-positive (CK+) cells by a Laser Capture Microdissection System for the determination of MAGE-1, -2 mRNA expression was used to detect ITC. This study shows that this method is highly sensitive as it allows to determine beta-actin-mRNA expression in a single CK+ cell. Using > or =5 CK+ cells as a cut-off level, the MAGE-1 mRNA expression was detected in 100% of CK+ cells in the peripheral blood and in 75% of bone marrow samples of patients with gastric cancer. The MAGE-2 mRNA expression was observed in 40 and 58% of samples, respectively. Furthermore, an analysis of primary tumours and locoreginal lymph nodes with respect to the mRNA expression of the two genes showed that MAGE-1 mRNA expression was detected in 88% of the primary tumours and in 67% of the lymph node samples, whereas the MAGE-2 mRNA expression was observed in 72 and 67% of the cases, respectively. Thus, the method described here allows the precise and sensitive determination of tumour-associated gene expression in single ITC present in the blood and bone marrow of patients with gastric cancer.