Hemoconcentration is a technique that involves selective removal of plasma water and some dissolved solutes by way of an ultrafiltration membrane. Hemoconcentrators (HC) are routinely used during pediatric cardiopulmonary bypass (CPB) to remove free plasma water and various inflammatory mediators. This paper examines two different commercial HCs, the Dideco DHF0.2 and the Minntech Hemocor HPH 400. Both products were evaluated when used for prebypass ultrafiltration (Pre-BUF), and postbypass modified ultrafiltration (MUF). Five HCs were evaluated for each product, and the two groups were compared during the two different phases of the cardiac procedure. During the pre-BUF period, both groups of HCs were tested at a transmembrane pressure (TMP) gradient of 500 millimeters of mercury (mmHg). The mean amount of ultrafiltrate (UF) removed by the Dideco DHF0.2 was 81.4 milliliters per minute (mL/min), and the mean amount of UF removed by the Minntech Hemocor HPH 400 was 90.8 mL/min during the pre-BUF period. During the peribypass period flow parameters were much harder to define. As a result, these data are not reported. During the MUF procedure, the mean amount of UF removed by the Dideco DHF0.2 was 74.2 mL/min at an average MUF flow rate of 130 mL/min. The mean amount of UF removed by the Minntech Hemocor HPH 400 was 81.4 mL/min at an average MUF flow rate of 127 mL/min. Both products performed adequately under the clinical circumstances described above. The Minntech HPH 400 produced a hemofiltrate that was consistently tinged with a slight red color. The Dideco DHF0.2 consistently produced a hemofiltrate that was noticeably clearer than that of the Minntech device.