Ultrafiltration is an extracorporeal technique that employs the principle of convective solution transport across a semipermeable membrane and by which plasma water is removed from the dilute blood during cardiopulmonary bypass. Ultrafiltration has been used in 40 pediatric cases who underwent corrective surgery for congenital heart disease (TOF 12, VSD 23, ASD 4 and PAVC 1) intraoperatively. The patients' ages ranged from 10months to 9 years (4.9±2.6yrs)and weights from 7 to 28kgs (16.4±5.8 kgs). The ultrafiltrator was interposed in the port of arterial filter of the CPB circuit and performed during the rewarming period. Ultrafiltration blood flow rate was 250-300 ml/ min and negative pressure was less than -250 mmHg. The average ultrafiltrability (total blood flow volume/ultrafiltration fluid volume) was 11.2%(10.7-12.1%). After ultrafiltration HCT increased from 16.1 %(15-18%) to 34.7%(32-40%). In the ICU, 5 cases (TOF 3, VSD 1 and ASD 1) had hemoglobinuria. The indications of ultrafiltration in pediatric cardiac surgery are: 1. The initial perfusion is conducted with bloodless priming or with blood priming when the HCT value dropped to lower than 17% during CPB. 2. The CPB time is longer than 2 hours. 3. The patient receives digoxin and diuretics preoperatively or has complex anomals. Our experience showed that ultrafiltration during pediatric CPB is facile, safe, and effective. It may result in a reduced amount of fluid accumulation in extracellular fluid space with hemodilution perfusion which benefits the patients postoperative recovery. Hemoconcentration of dilute blood during cardiopulmonary bypass(CPB) by plasma water removal is an extracorporeal technique application by ultrafiltration during the pediatric open-heart surgery. Usually after CPB, especially in infant cases, the residual pump blood volume is much more than the acceptable volume for postoperative transfusion. Therefore, the transfusing of large volumes of dilute blood with low colloid oncotic pressure to the patient may cause circulatory overload with normovolumic anemia and result in significant fluid diffusion into interstitial space. In order to save the residual pump blood and reduce such ill effects and complications, we applied ultrafiltration on 40 pediatric cases during open-heart surgical procedures. Our clinical data, ultrafiltration application indications and recommended procedures are now presented.