The etiology of human breast cancer is unknown; accepted risk factors, e.g., menstrual, reproductive, and family histories, are implicated in less than half of all cases. Various halogenated hydrocarbons--acting as either co-carcinogens or promoting agents--which are derived from the environment and are concentrated in human fatty stores, may also play a role in breast cancer risk. A pilot study was undertaken to measure and compare levels of chemical residues in mammary adipose tissue from women with malignant and nonmalignant breast disease. Elevated levels of polychlorinated biphenyls, bis (4-chlorophenyl)-1,1 dichloroethene, and bis(4-chlorophenyl)-1,1,1 trichloroethane were found in fat samples from women with cancer, compared with those who had benign breast disease. These results, although preliminary, suggest a role for environmentally derived suspect carcinogens in the genesis of mammary carcinoma.