The aim of this study was to determine whether BRCA1 and BRCA2 mutation carriers have different baseline CA125 levels compared with non-carriers, and whether a significant difference in pre- and post-operative CA125 levels exists in BRCA mutation carriers undergoing risk-reducing bilateral salpingo-oophorectomy (RRBSO). The study also considered whether CA125 measurements should continue in unaffected BRCA mutation carriers after RRBSO. 383 Eligible women were identified through retrospective review of the BRCA Carrier Clinic at The Royal Marsden NHS Foundation Trust, London, UK. These women all had CA125 levels measured as they were either a carrier or at risk of a BRCA1 or BRCA2 mutation. Of these, 76 went on to have a negative predictive test for their familial mutation and so are classed as 'non-carriers'. 133 BRCA1 and 87 BRCA2 carriers had RRBSO, with a further 26 BRCA1 carriers, 28 BRCA2 carriers and one non-carrier developing ovarian cancer. The remaining 21 BRCA1 and 28 BRCA2 carriers did not have RRBSO or develop ovarian cancer in the time of study follow-up. CA125 levels were measured as surveillance or as part of pre-RRBSO care. CA125 measurement post-RRBSO was continued in 48 BRCA1 and 40 BRCA2 carriers. In 154 BRCA1 mutation carriers, the median baseline (i.e. before RRBSO and with no clinical signs of ovarian cancer) CA125 level was 9.0 U/ml (range 2-78) and was 10.0 U/ml (range 1-43) in 115 BRCA2 mutation carriers. When compared with the 75 non-carriers (median baseline CA125 10.0 U/ml; range 2-52), there was no significant difference between the BRCA1, BRCA2 and non-carrier groups. There was a significant reduction in CA125 from pre- to post-RRBSO in 48 BRCA1 carriers (p = 0.04) but no significant difference in 40 BRCA2 mutation carriers (p = 0.5). Out of a total of 220 mutation carriers who underwent RRBSO, two had an incidental ovarian cancer found on histopathology and another developed primary peritoneal cancer during the follow-up period. Our study is the first to compare initial serum CA125 levels in BRCA1 and BRCA2 mutation carriers with those of non-carriers. Our study found no significant difference between the three groups. A drop in CA125 levels after RRBSO in BRCA1 carriers supports the finding of earlier studies, but differed in that the fall was not seen in BRCA2 carriers. The finding of only one case of post-operative peritoneal cancer in 220 carriers undergoing RRBSO supports the discontinuation of post-RRBSO serum CA125 monitoring in BRCA mutation carriers.