Objective To investigate the expression of ADAM9 in breast cancer and its clinical significance. Methods The expressions of ADAM9 in normal breast tissues and breast cancer tissues were detected by reverse transcription polymerase chain reaction (RT-PCR) and immunohistochemistry, and whose relationship with clinicopathologic features was analyzed. Results The expression of ADAM9 mRNA increased in the breast cancer tissues, but which was not detected in the normal breast tissues. The expression of ADAM9 protein in the breast cancer tissues was significantly higher than that in the normal breast tissues (Plt;0.05), and which in the metastatic lymph nodes was significantly higher than that in the negative lymph nodes or corresponding primary lesions (Plt;0.05). The expression of ADAM9 in the breast cancer tissues was correlated with the lymph node metastasis and histological grade (Plt;0.05). Conclusion ADAM9 is overexpressed in the breast cancer tissues, which might involve in the pathological progression of breast cancer.