Background and aims: Despite the endocrine system activity exhibited by polychlorinated biphenyls (PCBs), recent studies have shown little association between PCB exposure and breast cancer mortality. To evaluate the relation between PCB exposure and breast cancer incidence, a more sensitive endpoint than mortality, we studied women exposed to PCBs while employed in capacitor manufacturing facilities. Methods: We followed 5,754 women employed at least a year in three facilities in the United States, identifying cases via questionnaire, cancer registries, and death certificates through 1998. We collected lifestyle and reproductive information via questionnaire from participants or next of kin and used semiquantitative job exposure matrices (JEMs) for inhalation and dermal exposures combined. We generated standardized incidence ratios (SIRs) and standardized rate ratios (SRRs) and used Cox proportional hazards regression models to evaluate potential confounders and effect modifiers. Results: Overall, the breast cancer SIR was 0.81 (95% CI 0.72, 0.92, n=257) and regression modeling showed little effect of duration or cumulative exposure. However, for the 362 women of questionnaire identified races other than white, we observed positive, significant responses for duration and cumulative exposure; only smoking, birth cohort, and self or proxy questionnaire completion had significant explanatory power when added to models with exposure metrics. Discussion and conclusions: Overall, we found no elevation in breast cancer risk following occupational exposure to PCBs, but observed significant exposure-related risk elevations among non-white workers. The small number of cases (twelve) limits interpretation, but the finding warrants additional investigation, as the usual reproductive risk factors accounted for little of the increased risk.