Researchers at four clinical centers collected breast cancer data from 8,545 women in the study and followed them for an average of 3.2 years. They measured bone mineral density at the beginning of the study and collected information on breast cancer at year 1 and an average of 3.2 years later. The researchers excluded women who reported that they were on estrogen replacement therapy at the beginning of the study, leaving 97 confirmed breast cancer cases and 6,757 controls (women who did not develop breast cancer) for their analysis.

The investigators suggest that bone mineral density reflects a woman's lifetime exposure to estrogens, which is influenced by factors such as age at first menstrual period, age at menopause, and natural variations in estrogen levels among women, as well as estrogen replacement therapy. The implications of these findings for women are still unclear. More information is needed from large clinical studies of the effects of estrogen therapy on bone density and breast cancer, such as the Women's Health Initiative, to determine an overall strategy that provides the greatest health benefits and fewest risks for postmenopausal women.