After separation of pediatric patients from cardiopulmonary bypass (CPB), the authors salvaged red blood cells (RBCs) from the extracorporeal circuit by ultrafiltration and reinfused them to the patients. The purposes of this study were to determine 1) the effects of infusion of hemoconcentrated RBCs on hemoglobin, plasma free hemoglobin, and activated clotting time, and 2) the incidence of perioperative homologous RBC transfusion. Data were collected prospectively from 200 consecutive infants and children undergoing CPB during correction of congenital heart defects. The patients' hemoglobin, plasma free hemoglobin, and activated clotting time were measured both before and after infusion of 10 mL/kg of hemoconcentrate. Guidelines for intraoperative and postoperative transfusion of homologous RBCs were followed, and such transfusions were recorded. Significant increases in hemoglobin concentrations occurred when the hemoconcentrate was infused, as did statistically significant, but clinically manageable, increases in plasma free hemoglobin and activated clotting time. Perioperative homologous RBC transfusion was performed in 67% of patients (56% received intraoperative transfusion). Intraoperative transfusion was more frequent in small infants who were more hemodiluted by the clear CPB priming solution. Postoperative transfusion was more frequent in patients who had operation for cyanotic heart disease. Hemoconcentration by ultrafiltration after CPB is an effective and safe means of salvaging RBCs and reducing homologous RBC transfusion.