Objective: The aim of the present study is to explore the expression of a specific marker of breast cancer, small breast epithelial mucin(SBEM)mRNA, in peripheral blood and to investigate its significance. Methods: The expressions of SBEM-mRNA in peripheral blood of 67 patients with breast cancer, 16 patients with benign breast disease, and 20 normal healthy volunteers were detected with nested reverse transcription-polymerase chain reaction (Nested-RT-PCR). Results: SBEM-mRNA was negative in healthy individuals and patients with benign breast tumor, but 50.7%(34/67) of breast cancer patients showed positive expression of SBEM-mRNA in peripheral blood, of whom 25%(2/8) were in stage I, 45.8%(11/24) in stage II, 43.75%(11/24) in stage III and 73.7(14/19) in stage IV. The positive rate in stage IV was higher than that in stage I, II, III (P<0.05). Expressions of SBEM-mRNA had no correlation with age, carcinoma size, pathological type, ER and PR state (P>0.05). Conclusion: SBEM-mRNA is specifically expressed in breast cancer and it may act as a marker for the detection of micrometastasis of breast cancer.