Objective: The aim of this study was to detect micrometastases in bone marrow of primary breast cancer patients, and compare with other clinical parameters.Methods: Cytokeratin 19 (CK-19) gene mRNA expression was detected by reverse transcriptase-polymerase chain reaction (RT-PCR) and South blot hybridization.Human breast cancer cell line T47D was mixed with bone marrow cells in different proportions. The positive detection rate was compared among RT-PCR, South blotting and immunohistochemistry (IHC) methods. Results: Cytokeratin 19 gene was expressed in all 6 positive control samples while the expression was not seen in 8 negative control samples. In all 54 patients 14 cases were CK-19 positive (25.9%) by RT-PCR, another positive signal was obtained in 5/54 (9.3%) of bone marrow samples by South blotting. The total positive cases are 19/54 (35.2%).CK-19 IHC+ cells were detected at a dilution of one T47D cell in 5×104 bone marrow cells, while the sensitivity detected by PCR and South blot hybridization was at 1:5×105 and 1:1×106, respectively. This demonstrates that RT-PCR and South blotting was at least 20 times more sensitive than the IHC method. The micrometastases positive rate of the larger tumor size group (＞5.0 cm) was significantly (P＜0.05) greater than that of the smaller tumor size group (0-2.0 cm). Conclusion: detection of micrometastases in bone marrow by RT-PCR and South blotting, using CK-19 as a biological marker, is highly sensitive and it is a method to be used for anticipating the prognosis of breast cancer patients.