An experimental study of the wet-out or the shell-side of a commercial hollow fiber hemodialyzer is presented. Using X-ray Computed Tomography, we followed the movement of water into the shell of an initially dry unit. Local values or fiber packing fraction were also determined from the dry images. During wet-out. water moved preferentially through regions of lower fiber packing. Additionally, air pockets were observed in regions of higher fiber packing that were bordered by regions of lower flier packing. Thus, hydraulic permeability differences, due to non-uniform fiber packing, provide one mechanism for air entrapment.