Background: Breast cancer with hormone receptors (HRs) negative and human epidermal growth factor 2 (HER2) negative, so called triple negative subtype (TN), has attracted attention due to its poor clinical outcome.Invasive apocrine breast carcinomas are frequently TN, although they have relatively better prognosis than common type of invasive ductal carcinoma.It was also reported that positive expression of ERβ were frequently observed in the apocrine carcinoma.We recently reported that expression of ERβ in HR-positive patients treated with adjuvant tamoxifen correlated to better outcomes.From these observations, we initiated the investigation exploring the possibility of ERβ as a favorable prognostic factor in TN invasive breast carcinoma.Material and Methods: Among the 1412 primary breast cancer patients who underwent operation in our institution from 1998 to 2005, 540 patients were evaluated HER2 status.The 62 of 540 cases were defined to be TN phenotype (cT1-3, cN0-3, cM0), and were enrolled to this study.Surgically obtained specimens or core needle biopsy specimens in the cases with neo-adjuvant chemotherapy were subjected to immunohistochemical staining with ER-β antibody (mouse monoclonal, PPG5/10), and evaluated by Allred scoring system (score 0-3; negative, 4 or more; positive).Correlation between expression of ER-β and other clinico-pathological factors were evaluated by chi-square test, and prognosis in ER-β positive group and negative group were compared by Kaplan-Meyer Analyisis.Results: The rate of ER-β positivity in TN subtype was 64.5%.ER-β positive group tended to have better prognosis in disease free survival, although it was not statistically significant (p=0.1909).Whereas in overall survival analysis, ER-β positive group is significantly superior to negative group (p=0.0474).Conclusion: In our analysis, expression of ER-β in TN breast cancer indicates the favorable prognosis.This observation might be due to the consequence of favorable nature in TN cancer with ER-β expression.