We conducted a comprehensive study on the application of ultra-high-strip-density (UHSD) grids to mammography for the improvement of image contrast. These UHSD grids have strip densities of 70 to 100 lines/cm. After investigating the performance of mammographic grids with various design parameters through Monte Carlo simulation studies, we made prototypes of UHSD grids having lead strip thicknesses of 20 to 37 micron, aluminum interspacer thicknesses of 80 to 120 micron, and grid ratios of 2 to 6. The UHSD grids can be placed inside the cassette without a Bucky tray and thus will not increase geometric unsharpness. The measured physical characteristics and the phantom images obtained confirmed our findings in the simulation studies. A 40 to 90% increase in contrast was achieved at Bucky factors of 2 to 3.5; these results are comparable with those from conventional antiscatter grid techniques in mammography. The potential usefulness of the UHSD grids was demonstrated in a preliminary clinical comparison of mammograms.