Mammographic parenchymal patterns have been proposed as a method of determining women at high risk of developing breast cancer. Wolfe's original report of relative risks as high as 37:1 for "dysplastic" breasts (DY) as compared with adipose breasts (N1), with intermediate values of P1 and P2, were not uniformly confirmed by others. (Relative risks are used here as the equivalent of odds ratios.) A case-control study of 706 breast cancers, each with two matching controls, drawn from 40,000 participants in four Breast Cancer Detection Demonstration Project clinics, was conducted to assess the role of the Wolfe classification of breast parenchymal patterns as a breast-cancer risk factor together with a set of well-established risk factors for breast cancer. Relative risks of 3.1 for DY to N1, 3.5 for P2 to N1, and 2.0 for P1 to N1 were determined. These are comparable to or greater than other known risk factors found in the same population. The Wolfe classification of parenchymal patterns strengthens the basis for clinical judgment, but should not be used exclusively to determine intervention in an individual patient's care.