(ABC News link)
Researchers at the Fred Hutchinson Cancer Center in Seattle followed 35,016 post-menopausal women who had no history of breast cancer.
They asked the women to fill out a questionnaire about their use of non-vitamin, non-mineral specialty supplements, and then followed their progress for an average of six years. They found that women who regularly used fish oil supplements had a 32 percent reduced risk of developing the most common form of breast cancer. Of the total number of women in the study, 880 developed the disease.