Abstract: Objective: To detect the expression of BCAR1/p 130 Cas protein in primary breast cancer and its antiestrogen resistance during endocrine therapy, and to investigate the relationship between expression of the BCAR 1 gene and the prognosis of breast cancer.Methods:A total of 67ER-positive primary breast can -cer patients were randomly recruited between 2000 and 2001. These patients received standardized therapy including endocrine therapy and were followed up for 5 years. We used immunohistochemistry to detect the expression of BCAR1/p 130 Cas protein in the breast cancer samples. We also analyzed the association be-tween BCAR 1/p 130 Cas protein expression and clinicopathologic features including tumor size, pathological type, differentiation degree, menstruation status, lymph node metastasis, TNM staging, and efficacy of tamoxi -fen for endocrine therapy. Results: Of the 67ER-positive breast cancer cases, 29died of disease recurrence or metastasis. The other38patients had no recurrence or metastasis. Of the29patients who died of recur -rence or metastasis, 25cases were BCAR1/p 130 Cas protein positive, accounting for86.2% . Of the 38pa-tients without disease recurrence or metastasis, there were only 8 patients with BCAR 1/p 130 Cas protein ex-pression, accounting for 21.1%. A positive correlation was found between BCAR1/p 130 Cas protein level and relapse-free survival ( χ 2 =27.935 ; P=0.000 ). A positive correlation was found between the expression of BCAR 1/p 130 Cas protein and differentiation degree ( r=0.270 , P=0.027 ). A positive correlation was also found between the expression of BCAR 1/p 130 Cas protein and protein expression of C-erbB-2 (r=0.492 , P=0.000 ) in ER-positive breast cancer tissues. BCAR1/p 130 Cas protein expression was correlated with menstruation status ( χ 2 =13.77, P=0.003 ). No correlation was found between the expression of BCAR1/p 130 Cas protein and tu- mor size (P=0.548 ), pathological type (P=0.177 ), lymph node metastasis ( P=0.720 ) or TNM stage ( P=0.524 ). Conclusion: Breast cancer patients with high BCAR1/p 130 Cas protein expression had higher risk of relapse or metastasis. Overexpression of BCAR 1/p 130 Cas protein is correlated with tamoxifen resistance. Fadrozol is the best choice for endocrine therapy in postmenopausal breast cancer patients with ER-positive tumors. The BCAR 1/p 130 Cas protein may play a role in the progression of breast cancer. Elucidation of this critical pathway will be helpful for the development of new treatment strategies for antiestrogen resistant breast cancer.