Continuous arteriovenous hemofiltration with or without countercurrent dialysis (CAVH[D]) improved pulmonary gas exchange in eight children with concomitant renal and respiratory failure. Fluid accumulation had increased patient weight to 65.2 +/- 18.4 (SD) kg before therapy. After 48 h of CAVH(D), weight was reduced to 60.3 +/- 15.5 kg (p less than .02). Similarly, PaO2/FIO2 improved from 137 +/- 99 to 207 +/- 83 (p = .009) with PEEP unchanged or decreased. In patients with net negative fluid balance, pulmonary artery wedge pressure decreased (from 21.3 +/- 3.8 to 14.8 +/- 5.4 mm Hg; p less than .05). Colloid osmotic pressure increased (15.2 +/- 4.6 vs. 21.4 +/- 4.7 mm Hg; p less than .001). BUN and serum creatinine were unchanged. Parenteral nutrition infused was 212 +/- 427 ml/day before CAVH(D), and 1928 +/- 567 ml/day during its use (p less than .0001). CAVH(D) in children with multiple organ failure allowed better caloric intake, and led to improvement in pulmonary gas exchange. We speculate that CAVH(D) improves pulmonary gas exchange by removal of body and lung water, or by enhancing clearance of mediators associated with pulmonary dysfunction.