This work analyzes the influence of the set of mammograms used in the training processes of a computer aided diagnosis system on the overall performance. We used the mammograms provided by the Digital Database for Screening Mammography, one of the most extended research database. The obtained results seem to suggest an effect on the performance values obtained in a CAD system with different database subsets. Therefore, in order to make valid comparisons between CAD systems, the specification of the mammogram set used to test the system is of the utmost importance.