550 Background: Breast cancers (BC) in women carrying mutations in BRCA1 gene are more frequently estrogen receptor negative than the nonhereditary BC. However, tamoxifen has been found to have a protective effect in preventing contralateral tumors in BRCA1 mutation carriers. The identification of the second human estrogen receptor, ERβ, raised a question of its role in hereditary breast cancer. Patients and Methods: Formalin-fixed, paraffin embedded breast cancer tissues from 46 patients with mutations in BRCA1 gene were used in this study. A mutation analysis was carried out by a multiplex PCR assay. Only 3 common founder mutations in Poland in the BRCA1: 5382insC, 4153 delA, C61G were tested. Immunostaining for ERα, ERβ and PgR (progesterone receptor) was performed using monoclonal antibodies against ERα, PgR (DakoCytomation), and against ERβ (CHEMICON). The EnVision detection system was applied. The study population comprised a control group of 65 BC patients operated successively in 2002. The data were analyzed using a nonparametric Fisher-Freeman-Halton test; the statistical significance was considered when p<0,05. Results: BRCA1 mutation carriers were more likely to have ERα-negative BC than those in the control group. Only 17% of BRCA1-related cancers were ERα-positive compared with 58% in the control group (p=0,0001). The expression of ERβ protein was observed in 37% of BRCA1-related tumors. The expression of ERβ protein in ERα-negative tumors was twice as frequent in the BRCA1-related group. This was particularly striking at the age ≥ 50 where 33% of BRCA1 mutation bearing patients were exclusively ERβ positive vs. 13% in the control group. The results obtained in a similar analysis for patients below 50 showed 25% and 16%, respectively. The expression of PgR was observed in 17% of BRCA1-related BC and in 68% of tumors in the control group. Conclusions: In BRCA1-associated tumors the expression of ERβ was higher than the expression of ERα. This may, in part, explain the protective effect of tamoxifen in preventing contralateral tumors in BRCA1 mutation carriers. Testing patients with BRCA1 mutation for the presence of both ERα and ERβ may help to identify those who might benefit from endocrine therapy. No significant financial relationships to disclose.