Extracorporeal circulation with a membrane oxygenator (ECMO) was used in 11 patients with acute respiratory insufficiency who did not respond to conventional treatment. By pass was veno-arterial in every case, seven times with femoral artery return, three times with axillary artery return, and once with both femoral and axillary return. Five patients died on ECMO. Six patients were taken off ECMO and two of them are long-term survivors. In nine cases ECMO allowed short-term control of respiratory failure. The respective roles of oxygen supply from ECMO and the haemodynamic changes incurred by its use are discussed. Although use of ECMO for long periods seems less hazardous now, present results are restricted by the lack of therapy for the underlying pulmonary lesions.