Purpose: BRCA gene mutations and elevated serum estradiol (E₂) are well-known risk factors for breast cancer. The aim of this study was to investigate the association between BRCA gene mutations and serum E₂ level. Methods: We measured baseline (menstrual cycle day 2–3) E₂ levels of 96 women with breast cancer who underwent BRCA testing. Results: The mean age, parity, and age at menarche did not differ between women with and without BRCA1/2 mutations. Basal serum E₂ level was significantly higher in women with BRCA2 mutations compared to women with BRCA1 mutations or without BRCA mutations (71.7 ± 41.6 vs. 45.5 ± 20.7 vs. 38.5 ± 12.6 pg/ml in BRCA2 mutation carriers, BRCA1 mutation carriers, and non-carriers, respectively, p value = 0.03). Women with BRCA2 mutations had 3.1 times as great risk for high basal E₂ level (>48 pg/ml) as women without BRCA mutations after adjusting for age and BMI (95 % confidence interval: 1.3, 7.6). BRCA mutation carriers with high serum E₂ level were significantly younger than the carriers with low serum E₂ level (31.4 ± 3.1 vs. 34.7 ± 4.9 years, p = 0.04). Conclusions: In this pilot study, we found an association between high basal serum E₂ levels and BRCA2 mutations. Our results suggest that increased production of E₂ may have a role in the pathogenesis of BRCA2-mutation-related breast cancer.