Eleven hypervolemic and oliguric children with low cardiac output after cardiac operations were treated by slow continuous ultrafiltration or continuous arteriovenous hemofiltration. A mean negative fluid balance of 1.63 +/- 0.37 ml/kg/hr (standard error of the mean [SEM]) significantly improved the hemodynamic status within 59 +/- 6.1 hours (SEM). Although the central venous pressure decreased significantly from 15.2 +/- 0.84 to 8.8 +/- 0.92 mm Hg (p less than 0.0001), the mean arterial pressure increased significantly from 41.5 +/- 2.54 to 53.5 +/- 2.21 mm Hg (p less than 0.001). In addition, pH increased significantly from 7.31 +/- 0.01 (SEM) to 7.43 +/- 0.001 (SEM) (p less than 0.001) and oxygenation index (arterial oxygen tension/inspired oxygen fraction) from 119 +/- 15.2 (SEM) to 214 +/- 27.0 (SEM) (p less than 0.001). Hemodynamic improvement during slow continuous extracorporeal fluid removal allowed a significant decrease of the catecholamine infusion rate. After normovolemia had been achieved, continuous arteriovenous hemofiltration had to be continued in four children because of persistent anuria. Eight patients could be weaned from artificial ventilation and vasopressor support. Two patients died without recovery of renal function and one with restored renal function. Slow continuous ultrafiltration and continuous arteriovenous hemofiltration improve the cardiovascular function in children with low cardiac output by optimizing the preload conditions of the failing heart. In addition, they improve acid-base balance and pulmonary gas exchange.