Data from the Minnesota Breast Cancer Family Study cohort (n=6,130 women) were used to examine the association of type II diabetes with mammographic percent density and incident breast cancer (BC). Exposures and outcomes were self-reported. The first set of analyses evaluated diabetes (DM) as a risk factor for breast cancer. A total of 403 women (6.6%) reported a diagnosis of type II diabetes and 333 women reported an incident breast cancer. Women who reported type II diabetes had an age-adjusted relative risk (RR) for breast cancer of 1.44 (95% CI 0.89-2.32) compared to those who did not. Adjustment for covariates strengthened the association (RR 1.61, 95% CI 0.98-2.62). Mammograms were retrieved on women over the age of 40 and percent density was estimated with a user-assisted thresholding program. Cross-sectional analyses revealed that mean levels of mammographic density were not statistically significantly different between diabetic and non-diabetic women. Results were similar within strata of menopausal status. Breast cancer cases with diabetes did not have a significantly higher percent density than cases without diabetes. Our findings suggest that breast cancer risk may be increased among women with type II diabetes, but that type II diabetes does not significantly influence mammographic breast density.