The environmental organochlorines 2,2-bis(p-chlorophenyl)1,1,1,trichloroethane (
DDT) and
polychlorinated biphenyls (
PCBs) have been implicated as potential causes of female
breast cancer. We continued follow-up of our 1997 case-control study nested in the Nurses' Health Study cohort, adding 143 postmenopausal cases and controls to the original 238 pairs, and examining specific PCB congeners for the first time. We measured plasma levels of 2,2-bis(p-chlorophenyl)ethylene (
DDE), the major metabolite of
DDT, and
PCBs prospectively, comparing women who were diagnosed with
breast cancer between 1 month and 4 years after blood collection with control women in whom
breast cancer did not develop. Median concentrations of
lipid-adjusted
DDE, total
PCBs, and PCB numbers 118, 138, 153 and 180, assessed individually, were similar among the cases and controls. The multivariate relative risk of
breast cancer for women in the highest quintile of exposure as compared with women in the lowest quintile was 0.82 for
DDE (95% confidence interval [CI]: 0.49-1.37) and 0.84 for total
PCBs (95% CI: 0.47-1.52), 0.69 for
PCB 118 (95% CI: 0.39-1.22), 0.87 for
PCB 138 (95% CI: 0.50-1.50), 0.83 for
PCB 153 (95% CI: 0.47-1.48), and 0.98 for
PCB 180 (95% CI: 0.55-1.75). Sub-group analyses were also performed. Overall, our results do not support the hypothesis that exposure to
DDT and
PCBs increases the risk of
breast cancer.