Dense mammographic patterns are a risk factor for breast cancer. In a recent case-control study, dense patterns were observed to be associated with a higher relative risk estimate for the more aggressive grade 3 tumors than for grade 1 or 2 tumors. The present study was designed to ascertain, by reference to odds ratio estimates of relative risk by tumor size and histologic grade, whether this was more likely to be due to increased innate predisposition of dense parenchymal patterns to grade 3 tumors, or to dedifferentiation of cases missed at screening due to dense patterns. If the observation were due entirely to dedifferentiation, then for a given size the relative risk of grade 3 tumors would be the same as for grades 1-2. Size-specific relative risks of grade 3 tumors were higher than those of grade 1-2 tumors. It is concluded that the majority of the enhanced risk of grade 3 tumors is due to enhanced innate predisposition to grade 3 tumors in dense breasts.