Abstract: Objective : To determine micrometastasis in peripheral blood of axillary lymph node negative breast cancer (ALNNBC) patients with a new specific breast cancer marker and to discuss the relations between micrometastasis and some clinical and pathological factors and prognosis. Methods :Blood samples were obtained from 62 patients with ALNNBC, of which 6 patients with breast benign disease and 6 healthy adult women were screened for human mammaglobin (hMAM) mRNA by a nested RT-PCR assay. The sensitivity was assessed by detecting MDA-M B415 cells in healthy peripheral blood at different concentrations. Results : The assay exhibited a calculated analytical lim it of one tumor cell per 106 mononucleus cells. None of the samples from the patients with breast benign disease and the healthy adult women was positive, whereas 20 (32.3%) of 62 samples from patients with ALNNBC were positive for hMAM mRNA. It was also found that the micrometastasis rate was rather high in the C-erbB2-positive group (P<0.01) while it didn't correlate w ith menopausal status, tumor size, histological type, ER status and p53. It was significantly discovered that the micrometastasis rate in the metastatic group was apparently higher than in free-disease survival group (P<0.01). Conclusions : hMAM is at present one of the most sensitive and specific marker for detecting micrometastasis in peripheral blood of breast cancer. Micrometastasis of ALNNBC has a close, relationship with CerbB2 and might be a predictor of prognosis.