OBJECTIVE Cardiopulmonary bypass induces respiratory dysfunction postoperatively, with activation of both the complement system and white cells implicated. This study compared the effects of bubble and membrane oxygenators for cardiopulmonary bypass in terms of respiratory dysfunction and markers of white cell activation and endothelial damage. METHODS Fifty patients undergoing cardiopulmonary bypass were randomly allocated either membrane or bubble oxygenation. Respiratory function was assessed serially by arterial oxygen tension and alveolar-arterial oxygen gradient. Complement activation was measured by serial sampling of serum C3a levels. White cell activation was assessed by serial measurement granulocyte elastase; other markers investigated were levels of thromboxane B2, von Willebrand factor and malondialdehyde. All sample measurements were made preoperatively, early and late during bypass, 4-6 h postoperatively and then on the 1st, 2nd and 6th postoperative day. All samples were corrected for haemodilution, and differences between groups tested non-parametrically. RESULTS In both groups of patients there was a highly significant fall (P < 0.001) in arterial oxygen tension accompanied by a highly significant rise (P < 0.0001) in aleveolar-arterial oxygen gradient at 18 h compared to preoperative values persisting until 6 days postoperatively. Levels of C3a increased significantly in both groups at 10 min post bypass, increased further at 60 min peaking at 4-6 h post bypass. Granulocyte elastase serum levels increased significantly at 10 min postoperatively in both groups compared to control levels, remaining elevated till 48 h, but returning to control levels by 6 days. There was a small difference (P < 0.04) between the groups at 4-6 h only. Levels of von Willebrand factor increased significantly at 60 min post bypass in both groups, remaining elevated 6 days postoperatively. Levels of malondialdehyde increased at 10 min post bypass, remaining elevated until 6 days post bypass. Thromboxane levels showed no significant changes. For all markers measured, there were no significant differences between the groups other than those already indicated. CONCLUSIONS This study demonstrated marked respiratory dysfunction, complement activation and white cell activation in patients undergoing cardiopulmonary bypass with either bubble or membrane oxygenators. There was marked variability in the response of individual patients with either oxygenation technique, but overall no significant differences between the groups.