BACKGROUND: Ultrafiltration is a method to reduce capillary leak and the accumulation of extravascular water associated with cardiopulmonary bypass(CPB). There are two techniques of ultrafiltration, conventional and modified techniques. The effect of the former is controversial. The author investigated the effect of ultrafiltration performed in pediatric open heart surgery in our institute. METHODS: The retrospective study was done in 18 children who received surgical correction of congenital heart disease associated with left to right shunt. Eight children undergone ultrafiltration(UF group) were compared with 10 children who did not receive ultrafiltration(non-UF group). Ultrafiltration was performed mainly during rewarming of CPB(conventional UF) with or without modified technique for about 5 minutes immediately after bypass. Hematocrit(Hct), mean arterial pressure(MAP) and platelet count, 24-h blood loss, the amount of transfusion, fluid requirement, time to extubation, duration of ICU stay and the postoperative hospital stay were compared between groups. RESULTS: The volume of ultrafiltrate was 149+/-88 ml and ultrafiltrate/total blood volume(UF/TBV) ratio was 8.6+/-5.3%. The increase of Hct(5.4+/-1.7%) and of MAP(14+/-5.9 mmHg) after rewarming in UF group were significantly greater than 1.5+/-1.7% and 4+/-8.9 mmHg in non-UF group, respectively(p <0.05). There were no significant differences in platelet count, 24-h blood loss, the amount of transfusion, fluid requirement, maximum body temperature, time to extubation, duration of ICU stay and the postoperative hospital stay between groups. CONCLUSIONS: Ultrafiltration performed during open heart surgery in children significantly increases Hct and MAP immediately after ultrafiltration, but does not affect 24-h blood loss, the time to extubation, duration of ICU stay and the postoperative hospital stay.