Experimental studies show that hormonal and nonhormonal activities of polychlorinated biphenyls (PCBs) are structure dependent, suggesting that the breast cancer risk associated with PCBs may vary according to specific PCB congeners. In 1994-1997, the authors conducted a case-control study of Connecticut women to investigate whether breast cancer risk is associated with body burden of PCBs and varies by PCB congeners. A total of 304 breast cancer cases and 186 controls aged 40-79 years were recruited into the study. Fresh breast adipose tissue was analyzed for PCBs. The age- and lipid-adjusted geometric mean tissue levels of total PCBs were not significantly different (p = 0.46) for the cases (478.6 parts per billion) and controls (494.1 parts per billion). The covariate-adjusted odds ratio was 0.7 (95% confidence interval: 0.4, 1.1) for all study participants when the third tertile was compared with the lowest tertile. No individual congeners or groups of congeners were associated with a significantly increased risk of breast cancer. Further stratification by type of breast disease; menopausal, parity, and lactation status; and body size also showed no significant association with body levels of PCBs. These results suggest that environmental exposure to PCBs may not substantially affect breast cancer risk.