Abstract: Objective: To evaluate the specificity, sensitivity and their clinical significance of CK20 mRNA expression in the peripheral blood and bone marrow of patients with gastric cancer.Methods: Expression of CK20 mRNA was investigated by RT-PCR in peripheral blood and bone mar-row in 49 patients with gastric cancer, 5 patients with benign gastric lesion and 5 healthy volunteers.Results: The positive rate of CK20 mRNA were 49.0% (24/49) in bone marrow and 32.7% (16/49) in peripheral blood, with a significant difference between the two groups (P<0.05). There were 2 cases with negative results in bone marrow and positive results in peripheral blood. No CK20 mRNA expres-sion was found in the samples of patients with benign gastric lesion and healthy volunteers. The sensitivity of CK20 mRNA RT-PCR was 10^-5 in our study. The micrometastasis in peripheral blood and bone marrow was closely correlated with the histological differentiation and TNM staging. Conclusion:Using CK20 mRNA as a marker, the detection of micrometastasis with RT-PCR technique shows favor-able specificity. Bone marrow is more sensitive to detect micrometastasis than peripheral blood (one test), but the latter can still provide some valuable data.