The etiology of breast cancer is closely linked to the female hormone estrogen, with high life-time exposure being suggested to increase breast cancer risk [Nature 303 (1983) 767]. However, there appears to be a disparity between studies attempting to establish an association between high estrogen levels and breast cancer risk. This disparity becomes smaller by taking into consideration a timing factor, and we propose that estrogens can increase, decrease, or have no effect on breast cancer risk, depending on the timing of estrogen exposure. We further propose that the timing of estrogenic exposures may play at least as important a role in affecting breast cancer risk as life-time exposure.