OBJECTIVE Open-heart surgery without homologous blood transfusion is still difficult in children because priming volume in cardiopulmonary bypass circuit results in extreme hemodilution. Vacuum-assisted cardiopulmonary bypass circuit has the benefit of improving venous return and results in lowering priming volume. We introduced vacuum-assisted cardiopulmonary bypass circuit in order to reduce priming volume for pediatric patients in March 1995. A retrospective study was made on the efficacy of vacuum-assisted circuit for pediatric open-heart surgery in reducing homologous blood transfusion. METHODS Patients weighing from 5 to 20 kg who underwent surgery between January 1991 and June 1996 were divided into two groups, group A comprised 128 patients before introduction of this circuit and group B comprised 49 patients after introduction, and their clinical course was compared. Vacuum-assisted circuit was used in 27 patients of group B. RESULTS The percentage of transfusion-free operations was significantly higher in group B than in group A (33.6% in group A vs. 53.1% in group B, P = 0.014), and particularly this percentage in patients weighing less than 10 kg significantly increased (0% in group A vs. 42.9% in group B, P < 0.01). The amount of homologous blood transfusion was significantly lower in group B than in group A (374 +/- 362 ml in group A and 212 +/- 287 ml in group B, P < 0.01). The rate of complications and the duration of respiratory support did not differ between the two groups. The duration of hospital stay was lower in group B than in group A. CONCLUSIONS The findings of this study indicate that vacuum-assisted circuit is useful for pediatric open-heart surgery in reducing homologous blood transfusion.