Objective To explore the expressions of Smad4 and estrogen receptor (ER) and their interrelation,and the relationship with the clinicopathological features of breast cancer.Methods The immunohistochemical SP method was used to detect the expressions of Smad4 and ER in 50 case of invasive cancer,12 cases of carcinoma in situ and 15 cases of normal breast tissues.The differences in different clinical stages,differentiation degrees and nodal metastases were analyzed.The correlation between Smad4 and ER was explored.Results The positive expression rate of Smad4 in invasive cancer was 52.00％,which lower than that in normal breast tissue (93.33％),with a significant difference (x2 =8.329,P =0.004),positive expression rates of ER were 60.00％ and 40.00％ respectively,with no significant difference (x2 =1.868,P =0.172).The positive expression rates of Smad4 in carcinoma in situ and invasive cancer were 75.00％ and 52.00％ respectively,with no significant difference (x2 =2.082,P =0.149).The positive expression rates of ER were 58.33％ and 60.00％ respectively,with no significant difference (x2 =0.011,P =0.916).The positive expression of Smad4 was related to the TNM stage (x2 =6.392,P =0.011) and the lymph node metastasis (x2 =6.738,P =0.009),but it was not associated with the histologic grade (x2 =0.542,P =0.462).The positive expression of ER was related to the lymph node metastasis (x2 =4.133,P =0.042) and histologic grade (x2 =5.357,P =0.021),but it was not associated with the TNM stage (x2 =1.159,P =0.282).There was positive correlation between Smad4 and ER in breast cancer tissue (r =0.263,P =0.032).Conclusion Smad4 is expressed at lower level in breast cancer than in normal breast tissue.The expressions of Smad4 and ER are related to the different clinicopathological features of breast cancer with positive correlation.