A classification scheme to mammographically identify women at high risk for breast cancer proposes four patterns of breast duct prominence: N1 (normal), P1, P2, and DY (dysplasia). The authors investigated the claims of several studies that women who later get breast cancer are more likely than controls to be in the P2 and DY categories, and attempted to determine whether parenchymal patterns are related to known epidemiological risk factors. Mammograms of the noncancerous breast of 102 women with cancer were matched according to age and race with those of 204 controls; the two sets were intermixed and classified by parenchymal type. Patients and controls were similarly distributed by parenchymal type, the majority being P2 and DY. Nulliparous women and women with a family history of breast cancer were those most likely to fall into the P2 and DY categories. The risk estimates associated with these categories vary considerably across studies, and breast parenchymal patterns may be related to other known risk factors.