Six children with refractory heart failure were treated by continuous arteriovenous haemofiltration. The cause of the failure was postoperative fluid overload or low cardiac output with anuria or oliguria. This produced a mean (2 SD) negative fluid balance of 1.4 (0.6) ml/kg/h and reduced mean (2 SD) body weight from 4.7 (2.2) to 4.2 (2.3) kg over a period of 57.5 (31.1) hours. Central venous pressure fell significantly from 13.7 (3.1) to 7.7 (0.7) mm Hg while the mean (2 SD) arterial pressure increased significantly from 44.6 (5.5) to 52.6 (5.1) mm Hg. In three infants urine production resumed when normal blood volume had been achieved. The other three infants needed further haemofiltration because of prolonged renal failure. All but one was weaned from artificial ventilation and catecholamine treatment. No adverse haemodynamic effects were noted. One child need operation for a femoral artery thrombosis after 12 days of continuous arteriovenous haemofiltration.