Ultrafiltration hollow fiber modules have high packing densities, as the fibers are placed side by side very closely. This parallel packed configuration offers a resistance to radial flow in which the feed flow is on the outside and perpendicular to the direction of the fibers. A simple model describes the evolution of the pressure in a bundle of hollow fibers in dead-end filtration mode. The fluid flow is assumed to be laminar and follows a Blake-Kozeny type law. The theoretical radial pressure drops are markedly lower than those obtained experimentally. A first observation is that the bundle of fibers compacts under pressure. This compaction is linked to the non-rigid nature of the fibers and decreases the global porosity of the bundle, thus increasing the pressure drop. Other hypotheses, such as the non-regularity of fiber arrangement, qualitatively explain the difference between theory and experiment.