The correct operation of a mammography system's automatic exposure control system (AEC) is essential if mammograms are to be produced with a suitable film exposure. A methodology has been developed that allows the performance of the AEC to be assessed quantitatively using clinical films. By digitizing mammograms, the mean optical density (OD) in the main breast region and in a region of interest corresponding to the position of the AEC detector are evaluated for each film, together with the area of the main breast. Using these data it is possible to determine the relationships between the mean OD, breast size and AEC detector position. The performance of the AEC on a GE Senographe 600 TS system was investigated. The study found that there is a tendency to underexpose smaller breasts, i.e. with an area less than approximately 4000 mm(2). This is equivalent to a compressed tissue width of approximately 60---80 mm. The difference in mean OD between the mammograms of small and large breasts was up to 0.7 OD. Provided the sensitive area of the AEC detector is known, this method of assessing AEC performance can be used with any mammography system.