Fibers with elliptical cross-sections have higher surface to volume ratios than those with circular cross-sections and may therefore lead to increased filter collection efficiency. Single-fiber theory and velocity flow fields developed for elliptical fibers can be used to predict collection efficiency by diffusion for elliptical fibers. Utilizing the convective diffusion equation in elliptical coordinates, single-fiber diffusion efficiency was calculated for 4,312 combinations of cross-section aspect ratio, filter solidity, orientation of the cross-section to the air flow, and Peclet number. An empirical expression was developed from the results of these model runs to predict single-fiber diffusion efficiency for any combination of conditions. The equation indicates that diffusion efficiency is most strongly influenced by the Peclet number because decreases in particle size and increases in air velocity affect diffusion substantially. Increases in aspect ratio and solidity also increase the diffusion efficiency by making more fiber surface available for collection. Although the angle of orientation has the least effect of any of the factors, elliptical fibers with the major axis of the cross-section parallel to the incoming flow may have performance advantages over circular fibers if the angle of orientation can be controlled during filter production. This is because some elliptical fibers with the major axis parallel to the incoming flow have both higher single-fiber diffusion efficiency and less drag than circular fibers.