Digital mammography systems with detectors based on amorphous selenium exhibit outstanding spatial resolution characterized by the modulation transfer function (MTF). We measured the detector behavior of the Siemens Mammomat NovationDR with 70 μm pixel pitch and compared the results to analytical evaluations based on Monte Carlo simulations. Experimentally, the MTF of the mammography system is obtained from the images of a lead bar pattern or an edge phantom using different X-ray spectra. The simulations take into account all relevant X-ray interactions in the selenium layer. The resulting line-spread function is transformed to the MTF. Even at the Nyquist frequency (i.e., 7.14 mm-1), the measured MTF is well above 45% and thus fairly close to its theoretical limit (64%). The MTF shows a few percentage points of low-frequency drop, which can be explained in part by the presence of scattered radiation. The simulations allow the features observed to be explained. The detector investigated provides excellent spatial resolution and appears well suited for high-end mammography.