The efficacy of both preliminary and continuous hemodialysis during extracorporeal circulation (ECC) was evaluated in the pediatric population. Preliminary hemodialysis of the priming solution was initiated in eight children (HD-I group), while both preliminary and continuous hemodialysis during ECC was performed in another eight children (HD-II group). The control group which consisted of eight children did not undergo hemodialysis. Urine volume during ECC, perfusion pressure, and water balance were measured and compared among the three groups. In the HD-I and HD-II groups, the total urine volume during ECC was greater than in control group, but there was no significant difference. In the control group, the perfusion pressure temporally decreased after the pump was operating for five minutes, but the perfusion pressures remained stable in the HD-I and HD-II groups. Therefore, preliminary hemodialysis appears to prevent the "initial drop" seen in perfusion pressures. Water balance in the control group tended to be in positive balance, but both the HD-I and HD-II groups appeared to be in negative balance. Significant differences between HD-I and HD-II in contrast to the control group (p < 0.05) and p < 0.04, respectively) were observed. Preliminary hemodialysis is useful during ECC in infants and children because by maintaining perfusion pressure urine output remains adequate while a net negative water balance is achieved. Preliminary hemodialysis will increase the continuous hemodialysis in infants who are subjected to prolonged extracorporeal circulation.