Purpose: To investigate the expression of KAI1 protein and its clinicopathological significance in breast cancer. Methods: Immunohistochemical staining ( S-P method) was used to detect the expression of KAI1 protein in specimens from 107 breast cancer and 30 adjacent normal breast tissues. Results: KAI1 protein positive rate in breast cancer tissues is significantly lower than that of adjacent nomal breast( P 0. 05). Conclusions: These data suggest that in advanced breast cancer, KAI1 is down-regulated and decrease in KAI1 ecpression plays an important role in the malignant progression of breast cancer. Therefore, detection of KAI1 protein might be a potentially valuable indicator for staging human breast cancer and predicting prognosis.