The findings of a large prospective cohort study involving 9,856 women with BRCA1 or BRCA2 mutations indicate a 72% cumulative lifetime risk of breast cancer in those harbouring a BRCA1 mutation and a 69% risk for those harbouring a BRCA2 mutation. Futhermore, the cumulative lifetime risk of ovarian cancer was found to be 44% and 17% in those with BRCA1 or BRCA2 mutations, respectively. The risk of breast cancer increased further in line with the number of first-degree, or second-degree relatives already diagnosed with either BRCA1-mutant or BRCA2-mutant breast cancer. These findings provide further evidence that will guide the counselling and treatment of women harbouring these mutations.