We describe the key features of a pre-production, slot-scan digital mammography system. A number of these units have been used in clinical studies over the past year for the purpose of demonstrating their equivalence to the conventional film-screen devices. Since the clinical data has not yet been fully analyzed, it is not possible to make definitive claims. However, with hundreds of patients examined, the results appear to leave very little doubt the SenoScan digital mammography system will prove equivalent to the conventional technology. The detector developed for this system has a sensitive area 1.0 cm wide by 22 cm long. It is constructed by abutting four charge-coupled-device (CCD) chips, which are optically coupled to thallium-doped cesium iodide scintillator by means of a thin fiber optic plate. Scanning is accomplished by attaching the detector to a rigid arm that swings in an arc, with the axis of rotation collinear with the X-ray tube focal spot. The total scan time for the 30 cm image width is less than 6 s, with an effective exposure time of either 0.2 or 0.4 s. Two resolution modes are available: 0.054 mm or 0.027 mm square pixel size; in the latter mode both the image length and width are halved, as is the scan velocity, so that the scan time remains the same. To compensate for the low X-ray utilization efficiency of the slot geometry, a tungsten rhenium target X-ray tube is employed. It is rated at 8 kW on the 0.3 mm focal spot; when used with a heat exchanger, it has been found to provide the patient throughput needed in a busy clinical practice.