Prophylactic mastectomy (PM) is a risk-management option for women at high familial risk of breast cancer (BC). This study describes the PM experience of women enrolled in a large observational cohort study involving families with a history of hereditary breast cancer. Within 357 multiple-case BC families [119 (33%) BRCA1 or BRCA2 mutation positive], identified via family cancer clinics, 49 cases of PM [21 (43%) BRCA1 or BRCA2 mutation positive] were identified and their clinical, pathological and genetic features reviewed. Families with at least one incidence of PM displayed stronger breast/ovarian cancer histories than did families without PM. Median age at time of PM was 45 years (range 28-58). Ten cases (21%) were bilateral PMs in unaffected women and 39 cases were contralateral PMs in women with prior invasive BC (71%) or ductal carcinoma in situ (DCIS) (8%). Most (88%) underwent total mastectomy. Unnecessary axillary surgery occurred in eight subjects (16%). Malignant histology was found in three PM specimens (6%). Prior to genetic testing, PM was performed in two women who were subsequently shown not to carry the mutation specific to their family. Optimal utilization of genetic testing to guide surgical decision making, appropriate surgical technique and careful pathology examination of PM specimens, are important issues to consider prior to PM in women at high familial risk of BC.