Monte Carlo simulations were used to quantify the amount of scattered radiation in a scanning slot detector geometry designed for use in digital mammography. Ratios of the scatter to primary (S/P) x-ray photon energy absorbed in the detector were obtained for a Lucite phantom, and were investigated as a function of photon energy, phantom thickness, and slot detector width. Over a Lucite phantom thickness range of 2–6 cm, the S/P ratios ranged from about 0.10 to 0.17 for a 4 mm wide slot detector at the x-ray photon energies used in mammography. These ratios increased by a factor of when the slot width was increased to 10 mm. In general, 20 keV photons gave S/P ratios similar to those of a 30 kVp x-ray spectrum (Mo Mo filtration). The use of a 3 cm air gap reduced the S/P ratios by a factor of between 2.5 and 3.4, depending on the phantom thickness. For a constant primary energy fluence, coherent scatter was reduced as photon energy increased, whereas Compton scatter increased with increasing photon energy. With no air gap, the contributions of coherent and Compton scatter were found to be equal at 25 keV, whereas the introduction of a 3 cm air gap resulted in equal contributions for the two scatter processes at 36 keV. A 10 mm wide slot detector consisting of a thick phosphor screen was compared to an ideal detector absorbing all incident primary/scatter photons. Average differences in the S/P ratios for these two detectors were ∼7% with no air gap and ∼4% with a 3 cm air gap. The results obtained in this study will assist in the design of an optimal slot detector for use in digital mammography.