Objective: We studied the alterations in the expression of estrogen receptor alpha (ER) in the progression from ductal carcinoma in situ (DCIS) to invasive ductal carcinomas (IDC). Methods: The mastectomy specimens of 120 cases containing both DCIS and IDC were examined. The expression of ER proteins were examined by immunohistochemistry. The difference of the expression of ER proteins between DCIS and IDC were compared. Results: There were 58.33% of the cases with DCIS expressing ER proteins, and 40.00% of the cases IDC expressing ER proteins. There was a significant decrease of ER expression in IDC compared to DCIS (χ2 = 4.034, P = 0.045). Conclusion: These findings substantiate the notion that breast cancer progression is often associated with alterations in expressions of ER. The underlying mechanisms of these alterations need further investigation.