Tobacco smoke is an established carcinogen, but the association between tobacco smoking and cancer risk in
BRCAmutation carriers is not clear. The aim of this study was to evaluate prospectively the association between tobacco smoking and cancer incidence in a cohort of BRCA1and BRCA2mutation carriers. The study population consisted of unaffected BRCAmutation carriers. Information on lifestyle including smoking histories, reproductive factors, and past medical histories was obtained through questionnaires. Incident cancers were updated biennially via follow‐up questionnaires. Hazard ratios (HRs) and 95% confidence intervals (CIs) were estimated using time‐dependent Cox regression models. There were 700 incident cancers diagnosed over 26,711 person‐years of follow‐up. The most frequent cancers seen in BRCAmutation carriers were breast ( n= 428; 61%) and ovarian ( n= 109; 15%) cancer. Compared to nonsmokers, (ever) smoking was associated with a modest increased risk of all cancers combined (HR = 1.17; 95%CI 1.01–1.37). Women in the highest group of total pack‐years (4.3–9.8) had an increased risk of developing any cancer (HR = 1.27; 95%CI 1.04–1.56), breast cancer (HR = 1.33, 95%CI 1.02–1.75), and ovarian cancer (HR = 1.68; 95%CI 1.06–2.67) compared to never smokers. The associations between tobacco smoking and cancer did not differ by BRCAmutation type or by age at diagnosis. This prospective study suggests that tobacco smoking is associated with a modest increase in the risks of breast and ovarian cancer among women with BRCA1or BRCA2mutation.