Cardiopulmonary bypass in children is frequently associated with increased capillary permeability resulting in increased total body water, tissue edema, and organ dysfunction. Hemodilution also occurs and may require transfusion. Modified ultrafiltration utilizes the extracorporeal circuit to perform hemofiltration of the patient immediately following cardiopulmonary bypass and has been shown to significantly reduce total body water and to improve hemodynamics. The circuit contents can be concentrated to reverse hemodilution without transfusion. This report describes circuit modifications, management, and preliminary results of modified ultrafiltration in 23 infants and children following cardiopulmonary bypass.