Increased bone mineral density (BMD), as a marker of higher integrated estrogen exposure over time, could be an important risk factor for postmenopausal breast cancer. In the Study of Osteoporotic Fractures 8065 non-black women age 65 years and older were followed for an average of 3.2 years. There were 121 incident breast cancer cases. The age adjusted incidence rate/1000 person years of breast cancer was substantially higher among women with high BMD at several measured bone sites. There was approximately a 2-fold higher risk of breast cancer for women in the upper as compared to the lower 25th percentile of BMD. Considerable controversy exists about the association of hormone replacement therapy (HRT) and increased risk of breast cancer. In this paper we modeled the effects of selection for HRT, presuming that women with lower BMD would be more likely to be on HRT, then estimated the observed versus potential risk of breast cancer among HRT users as compared to nonusers. The model suggests that the potential risk of breast cancer associated with HRT could be greatly underestimated and that postmenopausal women with high BMD who are placed on HRT could have a substantially increased risk of breast cancer. This model of increased risk of breast cancer associated with BMD and HRT needs to be evaluated within clinical trials and larger observational studies that include measures of BMD.