Modified ultrafiltration (MUF) for cardiopulmonary bypass (CPB) in children decreases total body water and improves left heart function. We investigated the effects of MUF in pediatric open-heart surgery. Eighty-six patients less than 15kg who underwent radical cardiac operation were divided into four groups according to whether they received transfusion during the operation or not, and whether MUF was performed after completion of CPB or not. MUF group showed significantly higher hematocrit levels 20min after the end of CPB compared with non-MUF group in both transfused group and non-transfused group. Then, MUF group showed a significantly higher serum protein level than non-MUF group. In MUF groups, the systolic blood pressure elevated without the elevation of the left atrial pressure. We calculated PaO₂/FiO₂ as an index of postoperative lung function. The postoperative PaO₂/FiO₂ of MUF group was significantly higher than that of non-MUF group in transfused group. MUF significantly decreased homologous blood transfusion during the operation. MUF after CPB elevated hematocrit level and serum protein level, and improved cardiac function without volume load. Since MUF reduced the need for homologous blood transfusion. MUF is a useful means for pediatric cardiac surgery.