In open heart surgery in neonates and small children, the cardiopulmonary bypass circuit surface and the priming volume are relatively large in relation to patient size and blood volume. Therefore, the use of allogeneic blood is inevitable to maintain the optimal hematocrit level during bypass. To avoid the deleterious effects of blood transfusion, as well as to reduce the contact surface of blood with artificial materials, we stepwise reduced the bypass circuit size. Use of the commercially available minimized elements and an adjusted set-up of the system allowed us to reduce usage of allogeneic blood in the prime and during the bypass. However, other supplemental measures are needed to obtain asanguineous cardiopulmonary bypass for neonatal and infant patients.