The relationship of "high risk" mammographic patterns to histological risk factors for development of cancer in the human breast. In the UK Trial for the Early Detection of Breast Cancer in Nottingham, 119 women were identified as having fibrocystic change with epithelial hyperplasia or in situ carcinoma. Their mammograms were classified according to Wolfe's criteria and the corresponding histology for each patient was classified for degrees of epithelial hyperplasia, atypia and in situ neoplasia using Page's criteria. A control population of patients presenting for breast screening was used to represent the general population. No correlation was found between the four mammographic Wolfe patterns, N1, P1, P2 and DY and histological evidence of epithelial hyperplasia, atypia or in situ carcinoma. A further study was carried out to determine histological features of Wolfe pattern, using radiological examination of resected breast tissue. The variation in Wolfe pattern was related to the distribution of fibrous and adipose tissue in the breast interlobular stroma and appeared to have no relationship to epithelial parenchymal content. This information does not support the hypothesis that radiographic densities of P2 and DY patterns correspond to high risk epithelial proliferation. 