Objective To analyze the relationship of CAS protein expression with proliferative index, apoptosis index and clinical parameters in breast cancer tissues. Methods Immunohistochemistry for CAS expression, ki-67 (proliferative index) and TUNEL (apoptosis index) were examined in 20 usual ductal hyperplasia (UDH) , 20 atypical ductal hyperplasia (ADH), 10 ductal carcinoma in situ (DCIS), 53 invasive ductal carcinoma(IDC) and 14 normal breast tissues. Results CAS expression increased in order in the normal breast tissues, UDH, ADH, DCIS and IDC, with the positive rates of CAS protein being 14.3% (2/14), 25.0% (0/20), 40.0% (8/20), 60.0% (6/10), and 75.5% (40/53), respectively (χ2=29.382, P=0.000). CAS protein expression was correlated with histological grade, mitotic activity, and lymph node metastasis of IDC (P<0.01, P<0.05), and not with patient age, tumor volume or grade. CAS protein expression was positively correlated with ki-67 index (r=0.439, P=0.003), and not with the apoptosis index (r=0.248, P=0.083). Conclusion CAS protein expression is associated with cell proliferation index in breast cancer tissues.