To examine the association prospectively between endogenous hormones and breast cancer, a population-based, nested case-control study was conducted using serum collected in 1974. Serum hormone levels among 51 women, who subsequently developed breast cancer, were compared with controls matched on age and time since last menstrual period. The levels of estrogens, progesterone, sex-hormone binding globulin, and androstenedione were compared between cases and controls. No statistically significant differences in endogenous hormones levels were observed between women who subsequently developed breast cancer and controls. Despite the fact that risk factors for breast cancer implicated endogenous hormones, especially estrogen, in the etiology of this disease, our study failed to demonstrate a statistically significant association between endogenous hormones and the risk of breast cancer. If there is an association between endogenous hormones and breast cancer, the magnitude of the effect is weak.