PURPOSE: It is very important to detect hematogenous spread of cancer cells early in breast cancer patients in order to properly determine the prognosis and adjuvant therapy. In this study, we attempted to detect Cytokeratin-20 (CK-20) as a mRNA marker of cancer cells in peripheral blood and bone marrow using the reverse transcriptase-polymerase chain reaction (RT-PCR). METHODS: We obtained peripheral blood and bone marrow from 41 breast cancer patients who had been treated by Korea University Hospital. Using the RT-PCR method, we detected the CK-20 and compared the positive detection rate and concordance rate between two sources. Furthermore, we analysed the correlation with other known prognostic and predictive factors. RESULTS: The CK-20 detection rate was 36.5% in peripheral blood and in bone marrow. The concordance rate between both sources was 56%. In CK-20 detection, there was no significant correlation seen between peripheral blood and bone marrow (p>0.05). Additionally, there were no statistically significant correlations found between the other predictive factors (ER, PR, p53 protein expression, nm23 protein expression). Six cases who were CK-20 positive and were detected in both (peripheral blood, bone marrow) source showed advanced stage and axillary lymph node metastasis (p<0.05). CONCLUSION: The detection of CK-20 in peripheral blood and bone marrow correlated with stage and axillary nodal status. Therefore, this may suggest poor clinical prognosis if CK-20is detected in both sources in a breast cancer patient. The RT-PCR assay for detection of CK-20 is a very sensitive method, however the standardization and quality control of the RT-PCR method are important and multi-center trials are required.