In a nested case-control design in postmenopausal WHI participants, including women with incident TNBC (n = 637), incident HGSOC (n = 511), and matched cancer-free controls (n = 1841 and 2982, respectively), WBC BRCA1 promoter methylation was significantly associated with higher risk of incident TNBC and incident HGSOC. This evidence concerns the gene BRCA1 and cancer.