Background: Few studies have shown that the association between mammographic breast density and breast cancer persists for up to 10 years after the mammogram. We investigated associations of percent density, absolute dense, and nondense areas with breast cancer risk according to the time since the mammogram. Methods: This study included 1,028 incident breast cancer cases diagnosed within the Nurses' Health Study and 1,780 matched controls. Breast density was measured from digitized film images with computerized techniques. Information on breast cancer risk factors was obtained prospectively from the biennial questionnaires before the date of cancer diagnosis for cases and their matched controls. The data were analyzed with logistic regression. Results: Breast cancer risk increased with increasing percent density and increasing absolute dense area and decreased with increasing nondense area. In multivariate analysis, the magnitude of the association between percent density and breast cancer was similar when the time since the mammogram was <2, 2 to <5, and 5 to <10 years [density ≥50% vs.<10%: ORs, 3.12; 95% confidence interval (CI): 1.55-6.25, 5.35 (95% CI: 2.93-9.76), and 3.91 (95%CI: 2.22-6.88), respectively]. Similarly, the magnitude of association between quartiles of dense and nondense areas and breast cancer risk were similar across the time strata. We found no interactions between the time since the mammogram and breast density measures (Pinteraction > 0.05). Conclusions: Patterns of the associations between percent density, absolute dense, and nondense area with breast cancer risk persist for up to 10 years after the mammogram. Impact: Aone-time density measure can be used for long-term breast cancer risk prediction.