The expression of C-erb B-2 oncogene protein in the tissues of normal breast, atypical cystic hyperplasia of breast and breast carcinoma were determined in order to analyse the significance of the changes of C-erb B-2 oncogene in the course from atypical hyperplasia to carcinoma of breast. Results: there were no expression of C-erb B-2 protein in the tissues of normal breast and cystic hyperplasia of breast with epithelia proliferation grade Ⅰ. The positive rates of the expressions of C-erb B-2 protein in the tissues of atypical hyperplasia grades Ⅱ, Ⅲ and carcinoma of breast were 20.0%, 40.0% and 54.2% respectively. The results indicated that the abnormal changes of C-erb B-2 nocogene might occured in the carcinomatous transformation phase along with the changes from normal breast epithelial cell through atypical hyperplasia to breast cancer, those changes could be the result or promotion factor of abnormal proliferation of atypical hyperplasia of breast and was one of the markers of malignant transformation of epithelia of breast.