Epidemiologist Nancy Krieger and her colleagues at Kaiser and the Mt. Sinai School of Medicine in New York City studied 300 women from various ethnic groups, using frozen blood samples obtained when the women enrolled in the Kaiser program in the 1960s, before DDT was banned and the use of PCBs was restricted.

At least five small studies have attempted to link the chemicals, called organochlorines, with breast cancer, with mixed results. Among the most recent was a study of 58 women with breast cancer conducted by epidemiologist Mary Wolff of Mt. Sinai. Using blood samples obtained an average of six months before the diagnosis of breast cancer, Wolff found no link to PCBs, but a significant association with DDT.

That problem was eliminated in the Kaiser study, in which Wolff also participated, because the blood samples were collected, on average, 14.2 years before the cancer was diagnosed. Kaiser's large clientele also allowed the researchers to select a broader ethnic representation, more characteristic of the population at large, including 50 whites, 50 African Americans, and 50 Asians with breast cancer. They also identified an equal number of controls without the disease. In contrast, 80% of the women in Wolff's earlier study were white-a limitation Wolff acknowledges.