We have used ultrasound of the breast to define four parenchymal patterns in which increasing proportions of the breast are replaced by densely echogenic tissue. A series of 452 symptomatic women examined by both ultrasound and conventional X-ray mammography was reviewed to determine whether these ultrasonographic images could predict the breast parenchymal pattern defined mammographically. A very strong correlation was demonstrated between the breast pattern on ultrasound and the volume of the breast replaced by either dysplasia (Kendall's tau-b = 0.731 +/- 0.026, p less than 0.0001) or ductal prominence (Kendall's tau-b = 0.641 +/- 0.049, p less than 0.0001). This was seen both on initial reporting and on a blind re-reading of a random sample of 100 cases. The strength of correlation was similar for subgroups defined by family history of breast cancer, age, menopausal status, and history of benign breast disease, and the breast parenchymal pattern assessed by mammography or ultrasound showed similar associations with these variables. Ultrasonographic parenchymal patterns of the breast can predict the tissue patterns defined mammographically and may therefore be useful as a marker of breast cancer risk.