It is well established that the initiating event in chemical carcinogenesis is the binding of reactive carcinogens to DNA. Thus, a number of analytic methods have been developed for determining levels of carcinogen-DNA adducts in humans as a marker of individual exposure and, potentially, of risk for cancer development. We have developed monoclonal and polyclonal antibodies to carcinogen-DNA adducts and highly sensitive ELISA and immunohistochemical assays for determining levels of adducts in human tissues. These methods have been combined with genotyping and phenotyping methods for DNA repair to study gene-environment interactions in cancer risk. Recent studies on breast cancer have utilized two large biorepositories. The first is blood and tumor tissues collected as part of the Long Island Breast Cancer Study Project, a population-based case-control study of environmental risk factors. The second is the Metropolitan New York Registry of Breast Cancer Families, one of six sites funded by the National Cancer Institute as a part of the Breast Cooperative Family Registry (CFR). Analysis of samples from these two studies have demonstrated the utility of measurement of DNA adducts as biomarkers of exposure and that DNA repair capacity, measured by genotyping or phenotyping, can influence risk.