Hemodialyzers used for renal dialysis rely upon ultrafiltration to eliminate middle molecular-weight uremic solutes. It is valuable to measure the ultrafiltration rate locally to better understand dialyzer operation and to ensure there is no backfiltration of potential pyrogens into the patient's blood stream. We used a magnetic resonance flow imaging method to measure the local variation of the ultrafiltration rate along the length of a SYNTPA .160 hemodialyzer. The technique uses magnetic resonance fourier velocity imaging (MRFVI) to determine the percentage of flow in different velocity ranges. From these velocity profiles we estimate the maximum flow velocity in the lumen of the dialyzer fibers. Assuming laminar flow with a parabolic flow, profile we estimated the variation in the total volume flow, rate at each location where images are made. The local ultrafiltration rate declined uniformly over the length of the dialyzer, and the total rate is in agreement with independent measurements of ultrafiltration rate using pure water.