CONTEXT: Breast cancer (BC) is the most common cancer in women, with over 1 million new cases diagnosed every year worldwide. Over recent decades, considerable interest has emerged regarding whether vitamins and/or other supplements can lower the risk of BC. However, previous epidemiologic studies that investigated the association between intake of multivitamin and supplements of single vitamins and minerals and BC risk have reported conflicting results. Whether vitamins can actually reduce BC risk is still controversial. OBJECTIVE: This study examined whether multivitamin and calcium use was associated with BC incidence and DNA repair capacity (DRC). DESIGN: The research team designed an observational, case-control study. SETTING: All work was performed at the Ponce School of Medicine and Health Sciences under the direct supervision of principal investigator Dr Jaime Matta. PARTICIPANTS: Participants were 836 women recruited primarily from the private practices of oncologists, gynecologists, and surgeons in Puerto Rico. INTERVENTIONS: A total of 312 individuals in the breast cancer (BC) group and 524 individuals in the control group were compared for their multivitamin and calcium intake, DRC levels, and other covariates. OUTCOME MEASURES: Odds ratios (OR), adjusted using both crude analysis and multiple logistic regression, were used as measures of association between BC and DRC and other selected variables. RESULTS: The BC group had 30% reduced odds of taking multivitamins and calcium as compared to controls: (1) OR = 0.7 (95% CI, 0.4-1.0; P = .073) for multivitamins and (2) OR = 0.7 (95% CI, 0.4-1.2; P = .167) for calcium. Women with low DRC had 50% lower odds of taking calcium and 30% lower odds of currently taking vitamins OR = 0.5 (95% CI, 0.4-0.7; P = .001) for calcium and (2) OR = 0.7 (95% CI, 0.5-0.9.1; P = .047) for vitamins. CONCLUSIONS: Although this study is a case-control study in which the risk of BC could not be assessed, results suggest that vitamin supplementation could be an independent protective factor for BC. Calcium intake appears to affect DRC in a positive way, because it was associated with a high DRC level, which in turn is associated with low odds for BC.