Although use of dietary supplements among women with breast cancer is high, use among women at high risk of hereditary breast and ovarian cancer (HBOC) is unknown. This study assesses the prevalence of use of dietary supplements and identifies characteristics associated with use among women at high risk of HBOC who underwent genetic testing for cancer susceptibility. Participants were 303 women who underwent BRCA1/2 testing as part of Interdisciplinary Health Research International Team on Breast Cancer Susceptibility. Dietary supplements use was measured 12 mo post-disclosure. Potential determinants of use included personal cancer history, test result, psychological distress, cancer genetics knowledge, and health-related behaviors. Globally, 51% of participants used at least one dietary supplement. Calcium (26%), multivitamins (17%), vitamins D (14%), E (12%), and C (10%) were most frequently reported. Women > or = 50 yr were more likely to be using dietary supplements (P < 0.0001). Women with an inconclusive test result were more likely to use mineral supplements than noncarriers [odds ratio (OR) = 2.6; 95% confidence interval (CI) = 1.3-5.3]. Cigarette smoking was negatively associated with use of vitamin supplements (OR = 0.3; 95% CI = 0.1-0.7). Use of dietary supplements among women at high risk of HBOC who underwent BRCA1/2 testing is as frequent as use among patients with other types of tumors or use among individuals from the general population.