A CAD system for estimating the 3D (three dimensional) positions of lesions found in two mammographic views is described. The system is an extension of our previous method which finds corresponding 2D positions in different mammographic views by simulating breast compression [1]. In this paper, we first explain the principles and process flow of the system. The correctness of the 3D position calculated by the system is examined using breast lesions, which are found both in mammograms and in MRI data. Results from experiments show that the CAD system has clinical promise.