This research was performed to evaluate effects of filter resistance and housing geometry on the flow distribution through the filter and the resulting initial efficiency. Velocity distributions were measured in several housings with a Laser Doppler Anemometer. A simple filtration model was used with the measured velocities to predict efficiency distributions over the filter cross section. The results include measured velocity distributions, predicted local efficiency distributions, and overall filter efficiencies. The results show that filter resistance and housing geometry can have large effects on the flow field and significant, though lesser, effects upon installed filter efficiency, for both smaller and larger particles.