OBJECTIVE To test the hypothesis that bone mineral density (BMD) is associated with the risk of developing breast cancer in older women. METHODS Prospective cohort study with mean (SD) follow-up of 3.2 (1.6) years. METHODS Four clinical centers, one each located in the following areas: Baltimore, Md; Minneapolis, Minn; Portland, Ore; and the Monongahela Valley in Pennsylvania. METHODS A total of 6854 nonblack women who were 65 years of age or older and enrolled in the Study of Osteoporotic Fractures. METHODS Radius and calcaneus BMD by single photon absorptiometry at baseline; hip and spine BMD by dual-energy x-ray absorptiometry 2 years later. METHODS Breast cancer confirmed by medical record review. RESULTS A total of 97 women developed breast cancer. In the multivariate model, adjusting for age, the degree of obesity, and other important covariates, the risk of breast cancer was about 30% to 50% higher per 1 SD increase in BMD (relative risk, distal radius BMD=1.50; 95% confidence interval, 1.16-1.95). The age-adjusted incidence rate of breast cancer per 1000 person-years among women in the lowest quartile of distal radius BMD was 2.46, compared with 5.99 among women with the highest BMD. Women with BMD above the 25th percentile were at 2.0 to 2.5 times increased risk of breast cancer compared with women below the 25th percentile. Results were consistent across all BMD sites. CONCLUSIONS Bone mineral density predicts the risk of breast cancer in older women. The magnitude of the association is similar to that observed between BMD and all fractures. Our findings suggest a link between 2 of the most common conditions affecting a woman's health. Identifying a common denominator for these conditions should substantially improve our understanding of their etiology and prevention.