BACKGROUND. The Gail model has been commonly used to estimate a woman's risk of breast cancer within a certain time period. High bone mineral density (BMD) is also a significant risk factor for breast cancer, but it appears to play no role in the Gail model. The objective of the current study was to investigate whether hip BMD predicts postmenopausal breast cancer risk independently of the Gail score. METHODS. In this prospective study, 9941 postmenopausal women who had a baseline hip BMD and Gail score from the Women's Health Initiative were included in the analysis. Their average age was 63.0 ± 7.4 years at baseline. RESULTS. After an average of 8.43 years of follow-up, 327 incident breast cancer cases were reported and adjudicated. In a multivariate Cox proportional hazards model, the hazards ratios (95% confidence interval [95% CI]) for incident breast cancer were 1.35 (95% CI, 1.05-1.73) for high Gail score (≥1.67%) and 1.25 (95% CI, 1.11-1.40) for each unit of increase in the total hip BMD T-score. Restricting the analysis to women with both BMD and a Gail score above the median, a sharp increase in incident breast cancer for women with the highest BMD and Gail scores was found (P <.05). CONCLUSIONS. The contribution of BMD to the prediction of incident postmenopausal breast cancer across the entire population was found to be independent of the Gail score. However, among women with both high BMD and a high Gail score, there appears to be an interaction between these 2 factors. These findings suggest that BMD and Gail score may be used together to better quantify the risk of breast cancer. © 2008 American Cancer Society.