This prospective study was intended to determine in a homogeneous population of children whether hemofiltration, performed during cardiopulmonary bypass rewarming, is able to Improve hemodynamics and biologic hemostasis variables, to reduce postoperative blood loss, time to extubation, and plasma cytokines, and complement fragments. Thirty-two children undergoing surgical correction of tetralogy of Fallot were randomly assigned to a hemofiltration or control group. Hemofiltration was performed with a polysulphone hemofilter during rewarming of cardiopulmonary bypass. Plasma clotting factors, D-dimers, antithrom-bin-III, complement fragments C3a and C5a, interleukin-lβ, interleukln-6, interleukin-8, and tumor necrosis factor-a were measured before and after hemofiltration. Systemic mean arterial pressure, left atrial pressure, time to extubation, and postoperative blood loss were monitored. In the hemofiltration group, significant reductions in 24-h blood loss (250 (176–356) vs. 319 (182–500) ml/m2, median (minimum-maximum)), time to extubation (15 (9–22) vs. 19 (11–24) h), plasma concentrations of C3a, C5a, interleukin-6, and tumor necrosis factor-α were observed compared to control. Arterial oxygen tension on admission to the intensive care unit was significantly greater in the hemofiltration group (136 ± 20 vs. 103 ± 25 mmHg, mean ± SD). Significant increases in mean arterial pressure, clotting factors, and antithrombin-III were noted for the hemofiltration group. No intergroup difference was observed in left atrial pressure, platelets count, D-dimers, lnterleukin-8, and duration of stay in the Intensive care unit. Hemofiltration during cardiopulmonary bypass in children Improves hemodynamics and early postoperative oxygenatlon and reduces postoperative blood loss and duration of mechanical ventilation. Hemofiltration is able to remove some major mediators of the inflammatory response.