Both were suspect because they are chemicals in the environment with similarities to estrogen, the so-called female hormone associated with a risk of breast cancer.

The five studies were funded in 1993 by the National Cancer Institute and the National Institute of Environmental Health Sciences among women in the northeastern United States. None had shown a link between either DDT or PCBs and the Northeast's elevated rates of breast cancer. But some scientists thought the studies might simply have been too small and that their combined data might reveal such associations, at least for some subgroups of women.

Today that explanation was dashed as scientists analyzing the combined data also concluded that neither exposure explains the high rates of breast cancer in the U.S. Northeast. Their results appear in the May 16 issue of the "Journal of the National Cancer Institute".

The women in the five studies totaled 1,400 breast cancer patients and 1,642 controls. Two of the studies were conducted among women in New York state, one was in Connecticut, and one was in Maryland. Half the women in the fifth study, the nationwide Nurses Health Study, live in the northeastern states, including Maryland.

In each of the studies, blood was drawn from patients and controls alike and tested for DDE, the major break-down product of DDT, and for PCBs. DDT and PCBs were widely used in the United States until the 1970s and accumulate in the body's fatty tissues and thus can be found in human blood and breast milk many years after exposures.

The principal author of the analysis, Francine Laden, Sc.D of Brigham and Women's Hospital, Boston, said, "We found that the combined results from these five studies do not support an association between plasma or serum concentrations of DDE and PCBs and an increased risk of breast cancer."

The second author, Gwen Collman, Ph.D., of the National Institute of Environmental Health Sciences, said, "The investigators used a standardized approach to data analysis across all five studies and we did not find a consistent association in the various subgroups we looked at: Caucasian women, African-American women, women of various body mass and lactation histories."

Brigham and Women's Hospital is a 716-bed affiliate of Harvard Medical School. The National Institute of Environmental Health Sciences, while headquartered in Research Triangle Park, N.C., is a part of the National Institutes of Health, as is the National Cancer Institute. NIH and NCI have their headquarters in Bethesda, Md.