The radiographic appearance of the breast parenchyma provides a method of predicting who will develop a breast cancer. This paper describes a restrospective study of 7,214 patients. On the basis of the radiographic appearance of the breast parenchyma, patients were placed into one of four groups of risk for developing carcinoma of the breast. Follow-up studies revealed a stepwise progression in the incidence of developing carcinoma of the breast at least 6 months after the radiographic examination. In one of the two substudies, there was a 37 times greater incidence for those at highest risk compared to the low risk group. The classifications presented are thought to be of value in the everyday practice of mammography as well as in planning screening programs.