In this paper, we report the use of NMR (nuclear magnetic resonance) imaging for the investigation of flow profiles and flow-velocity distributions in hemodialyzers containing hollow-fiber membranes. For this purpose, we calculated a velocity encoding and velocity-compensated spin echo NMR imaging sequence. The performance of the sequence was tested on a simple flow phantom and the results were compared with simulated data showing good agreement regarding the geometry of the velocity profiles as well as the maximum flow velocities. Two different small-scale model hemodialyzers were investigated by means of NMR imaging to gather information about flow-velocity distributions in the two compartments of hemodialyzers. The results show that the incorporation of textile fibers as spacer yarn in one of these model hemodialyzers strongly narrowed the flow-velocity distribution in the dialysate compartment. Additionally, the spatial flow-velocity distribution was measured in a clinical-scale hemodialyzer to compare the results from model and clinical systems. Flow velocities were measured in the potting region of one of the model hemodialyzers and at a short distance downstream in the dialysate compartment, It was shown that swelling of the membranes in the potting region leads to higher flow velocities of the inner membrane flow.