Objective: Long-term extracorporeal lung assist is limited by a signi®cant mechanical blood trauma resulting in bleeding and hemolysis. To reduce the drawbacks of extracorporeal lung assist a new technique has been developed, by which the driving force for the extracorporeal circuit derives from the patients arterio-venous pressure gradient (pumpless extracorporeal lung assist). The aim of this clinical study was to test the feasibilty and effectiveness of pumpless extracorporeal lung assist in patients with acute respiratory distress syndrome. Methods: Twenty patients (41 ^ 16 years) with acute respiratory distress syndrome of various causes and failing respirator therapy were enrolled. The minimum hemodynamic requirements included a cardiac output (CO) . 6 l/min and mean arterial pressure (MAP) . 70 mmHg. Pumpless extracorporeal lung assist was established using a short circuit arterio-venous shunt including a special designed low-resistance membrane oxygenator which was placed between the patients legs. Results: At the time of inclusion FiO2 in all patients was 1.0 (paO2 45:9 ^ 7 mmHg, paCO2 58:9 ^ 17 mmHg). After 24 h of pumpless extracorporeal lung assist FiO2 was reduced to 0:8 ^ 0:1. A signi®cant improvement in oxygenation (paO2 84:1 ^ 21 mmHg, P , 0:05) and CO2 removal (paCO2 32:7 ^ 5 mmHg, P , 0:05) was notable. The mean extracorporeal ̄ow was 2:6 ^ 0:6 l/min, which represented approximately 25% of the patients mean CO (10:8 ^ 2 l/min). The median assist time was 12 ^ 8 (1±32) days. Fifteen out of twenty patients were weaned off pumpless extracorporeal lung assist. Five out of twenty patients died while on the system (four sepsis, one ventricular ®brillation). Three out of twenty patients died after successful weaning on day 8, 30, and 50, respectively. Twelve out of twenty patients were discharged in a healthy state (overall survival 60%). Technical problems included thrombosis of the venous cannula (n 5), thrombus formation within the membrane oxygenator (n 2), membrane oxygenator plasma leakage (n 2), and membrane oxygenator contamination with Candida albicans. No bleeding complication was observed. Conclusion: Pumpless extracorporeal lung assist is feasible and effective in a selected group of patients with acute respiratory distress syndrome but preserved hemodynamic function. By eliminating the pump and reducing the tubing length blood trauma can be minimized. Being very simple the system entails fewer risks of technical complications and also facilitates nursing care. q 2000 Elsevier Science B.V. All rights reserved.