Background: Polychlorinated biphenyls (PCBs), which are carcinogens and endocrine disrupting chemicals with innate heterogeneity, are hypothesized to influence risk of breast cancer. However, studies assessing PCB mixtures and breast cancer are lacking. Herein, we examined plasma levels of 17 PCBs as individual congeners and as mixtures in association with breast cancer. Methods: This study included 845 White and 562 Black women who participated in the case-control Carolina Breast Cancer Study Phase I. Cases (n=748) were women with a first diagnosis of histologically confirmed invasive breast cancer identified from the North Carolina (NC) Central Cancer Registry. Population-based controls (n=659) were identified from NC Department of Motor Vehicles or Health Care Financing Administration lists. PCBs were measured in plasma samples obtained at the time of the case-control interview. We used logistic regression to estimate adjusted odds ratios (ORs) and 95% confidence intervals (CIs) for the associations between individual lipid-adjusted PCB congeners and breast cancer. We used quantile g-computation (qgComp) to assess PCB mixtures, and in sensitivity analyses compared results to those from generalized weighted quantile sum regression (WQS). We examined effect measure modification by race (white vs. black). Results: The highest (vs. lowest) tertiles of PCBs 74, 99, 194, and 201 were associated with breast cancer ORs of 1.30 (95%CI=0.95-1.79), 1.39 (95%CI=1.02-1.89), 1.27 (95%CI=0.91-1.77), and 1.24 (95%CI=0.90-1.72), respectively. These PCBs also showed dose-response relationships is models using ln-transformed continuous PCB levels. Using qgComp, among all women a one-unit change in all PCB quantiles was associated with an OR of 1.29 (95%CI=1.00-1.67). In stratified analyses, associations of PCB mixtures were stronger among black women (ORqgComp=1.55, 95%CI=1.05-2.30), than among white women (ORqgComp=1.13, 95%CI=0.80-1.60). Conclusion: Exposure to PCBs may increase the risk of breast cancer. Additional studies are needed that consider mixtures of PCB congeners in association with breast cancer.