Patients with preoperative renal insufficiency are more likely to develop postoperative renal failure than those with normal preoperative renal function. Both of these groups may benefit from optimizing intraoperative renal perfusion because not all preoperative renal risk factors are easily diagnosed. Patients with preoperative chronic renal failure who are unable to manage perioperative electrolyte levels, excess water, and uremic toxins may benefit from intraoperative dialysis. Ultrafiltration is valuable in removing excess plasma water during CPB. Modified ultrafiltration studies suggest that ultrafiltration post-CPB can improve postoperative patient outcomes and that the mechanism for these improvements involve more than excess water removal. Since there are no contraindications for ultrafiltration or dialysis during CPB, the decision to use these techniques depends on the perceived potential benefits and the cost of adding a component to the CPB circuit.