The performance characteristics of a new dedicated mammography unit (a Mammo Diagnost U-M, Philips Medical Systems, Inc., Shelton, Conn.) are presented. The unit has capabilities for contact imaging with or without a grid and magnification imaging without a grid, using a molybdenum target and either of two different beam filtrations (molybdenum or aluminum). Characteristics evaluated include the performance of the beam restriction system, accuracy of technique parameter selector indication of kilovoltage, tube current and exposure time, focal spot size, exposure output and beam quality characteristics as a function of kilovoltage and filtration, compression device attenuation and automatic exposure control system performance. The resultant data have been used to calculate breast entrance exposure and average glandular dose for each imaging modality and to compare the different imaging modalities in terms of equivalent bandwidth, fe, using established models for the modulation transfer functions of the focal spot and image receptor. Modifications to the system to enhance clinical utility and to allow for routine calculations of patient exposure are also described.