Background: With open heart surgery(OHS), it has been recognized that many postoperative complications and postperfusion syndrome are associated with the activations of complements and leulocytes. Recently, some investigators also demonstrated that interlukin-6(IL-6) linked highly with postperfusion syndrome. The puropose of this study was to investigate the sequential changes of the IL-6 and to clarify each IL-6 relationship to the complements(C3, C4) and inflammatory response following cardiopulmonary bypass(CPB). Material and Method: To determine serum levels of IL-6, complements, leukocytes, and biochemistric markers of liver and renal function, blood samples were taken from th radial artery in 30 adult patients undergoing OHS with CPB. Result: Serum IL-6 levels incrased significantly at 10 minutes after CPB-on(CPB-10) in comparison with the control levels and reached the peak at CPB-off(p<0.05). Serum complement levels declined rapidly at CPB-10 and remained at the lower levels during CPB(p<0.01). Sequential changes of IL-6 levels had positive correlations with the changes of total leukocytes and neutrophil fractions(p<0.05), but had negative correlations with lymphocyte fractions(p<0.05). Changes of C3 related postively to monocyte fractions(p<0.05). Postoperative levels of total protein and albumin, decreased significantly in comparison with the control levels(p<0.01), while the postoperative levels of AST(aspartate transaminase) and bilirubin increased (p<0.01). At CPB-off, IL-6 levels had negative correlations with total protein and albumin levels(r=-0.60, -0.47 respectively, p<0.05), whereas C3 levels had positive correlations with albumin levels(r=0.40, p<0.05). IL-6 levels, as well as neutrophil fractions, had positive correlations with aortic clamp time(ACT) and total bypass time(TBT) (IL-6; r=0.82, 0.79 respectively, neutrophil fractions; r=0.50, 0.56 respectively, p<0.05), wheres lymphocyte frations and albumin levels had negative correlations whith ACT and TBT(lymphocyte fractions; r=-0.52, -0.58 respectively, albumin; r=-0.58, -0.55 respectively, p<0.05). Conclusion: These data showed that elevated production of serum IL-6 during CPB may play a pivotal role in systemic inflammatory responses and prologed CPB period may be assosiated with more sever postperfusion syndromes.