Risk reducing bilateral salpingo-oophorectomy (RRBSO) is highly effective for the prevention of high grade serous ovarian cancer (HGSOC) in BRCA1/2 pathogenic variant carriers (PVCs), but does not completely eliminate future risk of primary peritoneal cancer (PPC). The requirement to completely remove fallopian tubes at RRBSO and carefully exclude occult cancer/serous tubal intraepithelial carcinoma (STIC) lesions may not have been appreciated historically. We calculated rates of HGSOC and PPC in confirmed BRCA1/2 PVCs registered on the regional database in those who did (cases) and did not (controls) undergo RRBSO after genetic testing. Expected annual rates of ovarian/peritoneal cancer were 1% for BRCA1 ≥35yr and 0.5% for BRCA2 ≥45yr. Follow up before 35/45yr was ‘risk free’ and lead time excluded RRBSO <35yr and <45 years for BRCA1 and BRCA2, respectively. Women were followed from personal mutation report (controls) or RRBSO (cases) to death, ovarian/peritoneal cancer, or last follow up, whichever was sooner. In total, 891 cases (BRCA1=468, BRCA2=423) and 1,302 controls had follow up ≥35yr (BRCA1=736) and ≥45yr (BRCA2=566), respectively, over a total of 7,261.1 risk eligible years (mean=8.15yr). Twenty-one occult ovarian cancers were found at RRBSO (2.4%), 16 at stage 1. Post RRBSO, 56.97 ovarian/peritoneal cancers were expected but only 3 were observed (HR=0.053;95%CI=0.013-0.14), with combined Kaplan-Meier analysis HR=0.029 (95%CI=0.009-0.100,p<0.001). Risk reduction was greater in specialist (HR=0.03;95%CI=0.001-0.13) compared to non-specialist centres (HR=0.11;95%CI=0.02-0.37)(p=0.07). In controls, 23.35 ovarian/peritoneal cancers were expected with 32 observed (HR=1.37;95%CI=0.95-1.91). RRBSO <35/<45yrs reduces the risk of ovarian/peritoneal cancer by 95% in BRCA1/2 PVCs and may be greater in specialist centres. This article is protected by copyright. All rights reserved.This article is protected by copyright. All rights reserved.