The purpose of this study is to determine the prevalence and predictors of contralateral risk-reducing mastectomy (CRRM) in Australasian women at high familial risk of a second primary breast cancer (BC). Participants were women with unilateral BC and a strong family history of the disease, including BRCA1/2 mutation carriers. Data were collected through interview, self-administered questionnaire and review of pathology and surgical reports. Associations between CRRM and potential predictors were assessed using multivariate logistic regression. Of 1,018 women (median follow-up 11.1 years), 154 (15%) underwent CRRM, 43% of these within 12 months of initial BC surgery. More likely to undergo CRRM were women who were younger at BC diagnosis (odds ratio [OR] = 0.94 per year of age, P < 0.001), were diagnosed more recently (OR = 1.16 per calendar year, P < 0.001), underwent mastectomy as initial definitive BC treatment (OR = 5.2, P < 0.001) and underwent risk-reducing salpingo-oophorectomy (OR = 3.4, P < 0.001). BRCA1/2 mutation status, axillary nodal status and receipt of chemotherapy were not independently associated with CRRM uptake. A contralateral BC event (invasive or in situ) occurred in 177 (20.5%) of the 864 women who did not have CRRM, compared with one chest wall event (0.6%) in the 154 women post-CRRM. The contralateral event rate was 15.1 per 1,000 women-years for non-CRRM women and 0.7 per 1,000 women-years for CRRM women; P < 0.0001. Younger women with more recently diagnosed BC treated with mastectomy are more likely to elect CRRM. Neither BRCA1/2 mutation status, nor the competing risk of BC recurrence and death, appears to influence decision making.