The purpose of this study is to estimate the age-specific annual risks of breast cancer in a woman with a germline BRCA mutation and an affected first-degree relative according to the age of breast cancer diagnosis in the relative. Women with BRCA mutations with no previous diagnosis of breast cancer and with one first-degree relative with breast cancer were followed for breast cancers for a mean of 5.9 years (minimum 2 years). Age-specific annual breast cancer risks were calculated, according to the age of breast cancer diagnosis in the proband and the first-degree relative. 1114 cancer-free women with a BRCA mutation with a single first-degree relative with breast cancer were eligible for the study. 122 women (11.0 %) were diagnosed with incident breast cancer. The annual risk of breast cancer was 2.0 % for women with BRCA1 mutations and was 1.6 % for women with BRCA2 mutations. The age of breast cancer diagnosis in the first-degree relative did not affect the annual breast cancer risks for BRCA1 mutation carriers. For BRCA2 mutation carriers, the annual breast cancer risk was 4.5 % for women with a first-degree relative diagnosed with breast cancer under the age of 30 years and was 0.7 % for women with a relative diagnosed over the age of 60. Among women with BRCA2 mutations, a family history of early-onset breast cancer is a risk factor for developing breast cancer. Risk assessment for healthy BRCA2 mutation carriers should consider the ages of breast cancers diagnosed in first-degree relatives.