We are developing a dose-efficient system for digital mammography. The system incorporates a refractive x-ray lens combined with a photon counting silicon sensor system. The system matches the digital nature of x-rays and maximum information can be extracted from the transmitted x-ray beam. The photon counting sensor system opens the possibility to weight each photon according to its information content. High- energy photons with little contrast information can be weighted low while low energy photons are weighted high. At the same time, using the refractive x-ray lens, the incident x-ray spectrum can be tuned to the optimum energy for a certain breast thickness. The performance of the system is expected to be close to the theoretical limit in terms of dose efficiency and is entirely limited by quantum noise. Since the standard use of the term DQE does not take into account the incident x-ray energy spectrum, scattered x-rays or the difference in information content from low to high-energy photons an alternative measure of performance is used. We refer to this measure as dose-efficiency and in this case the system is related to an ideal x-ray imaging system.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.