Personal experience is reported on the use of a membrane oxygenator, the Capiox II, which is clinically compared with a bubble oxygenator for medium-term perfusion. The characteristics considered were the efficiency of the heat exchanger, the oxygenating capacity, traumatic effects on the blood and the direct effect on the renal and cardiopulmonary systems. The Capiox II demonstrated a better oxygenating capacity, less platelet damage, a smaller variation in the free plasma haemoglobin, a significant difference in postoperative bleeding and blood transfusion requirements, but showed no variation in renal and cardiopulmonary function. The authors suggest that the Capiox II is to be preferred for extra-corporeal circulation of medium duration, for which it combines the advantages of both bubble and membrane oxygenators, while being less complex and costly than previously marketed membrane devices.