A major obstacle to our understanding of the etiology of breast cancer is the lack of studies with prediagnostic serum for analysis. This year, the Cancer Prevention Studies Branch assumed responsibility for the Columbia, Missouri collection of the Breast Cancer Serum Bank from the Division of Cancer Biology, Diagnosis, and Centers, NCI. This collection includes serum collected between 1977 and 1987 from 7,641 women, 112 of whom subsequently developed breast cancer. Questionnaires on reproductive history, use of exogenous estrogens and other medications, and cancer diagnoses and treatment were completed at each blood drawing. We currently are evaluating estrogen and androgen levels in serum from 74 women who developed postmenopausal breast cancer and 148 matched controls. A study to evaluate pesticide and PCB levels in serum from cases and noncases is being developed and a study to analyze antioxidants is planned. Analyses of serum from the Breast Cancer Serum Bank will improve our understanding of the role of diet and hormones, as well as environmental exposures, in the etiology of breast cancer.