1513 Background: Next-generation sequencing enables rapid analysis of many inherited cancer susceptibility genes. Little is known about the prevalence and penetrance of pathogenic variants (PVs) in such genes among post-menopausal women with breast cancer, who comprise the majority of all breast cancer patients. Methods: The Women’s Health Initiative enrolled post-menopausal women from 1993-1998. We conducted a nested case-control study using banked DNA samples of 2,195 women who subsequently developed invasive breast cancer (cases) and 2,322 cancer-free controls. Sequenced genes were APC, ATM, BARD1, BMPR1A, BRCA1, BRCA2, BRIP1, CDH1, CDK4, CDKN2A (p16INK4a and p14ARF) , CHEK2, EPCAM, GREM1, MLH1, MSH2, MSH6, MUTYH, NBN, PALB2, PMS2, POLD1, POLE, PTEN, RAD51C, RAD51D, SMAD4, STK11, and TP53. PV were defined using American College of Medical Genetics criteria. PV prevalence is reported as proportions and penetrance as the odds ratio (OR) and 95% confidence interval (CI) of PV versus none among cases versus controls. Results: Among cases, the median age at diagnosis was 73 years; 66% were White, 18% Black, 6% Hispanic, 6% Asian and 4% other. The prevalence of PVs in any gene was significantly higher in cases (6.61%, 95% CI 5.57-7.65%) versus controls (4.09%, 95% CI 3.29-4.90%). The prevalence of BRCA1/2 PVs was 1.2% in cases and 0.22% in controls. Among cases, the prevalence of PVs in other breast cancer-risk genes was 2.3% ( ATM, CDH1, BARD1, BRIP1, CHEK2, NBN, and PALB2 collectively), two-fold higher than PVs in BRCA1/2. Prevalence of BRCA1/2 PVs decreased with age among cases, while prevalence of ATM, CHEK2 and PALB2 PVs did not. Statistically significant ORs for breast cancer penetrance were observed for BRCA1 (5.43, 95% CI 1.19-51.52), BRCA2 (4.71, 95% CI 1.84-15.08), BARD1 (9.78, 95% CI 1.04-1295.87) and PALB2 (6.30, 95% CI 1.93-31.94). Conclusions: Approximately 7% of women diagnosed with post-menopausal breast cancer carry a PV in a cancer susceptibility gene. In contrast to studies of younger breast cancer patients, PVs in other breast cancer-related genes were two times more common than in BRCA1/2. Results may guide genetic testing of women with post-menopausal breast cancer.