This prospective study was designed to elucidate the duration of systemic inflammatory response syndrome (SIRS) and the mechanisms that lead to the protraction of SIRS in patients who are operated on under cardiopulmonary bypass (CPB). The duration of SIRS in 13 patients with SIRS was studied. Two groups were divided according to the duration to investigate the meaning of the duration of SIRS. The perioperative parameters which significantly correlated with the duration of SIRS, including the kinetics of cytokines and white blood cells (WBC) counts were investigated. In patients with SIRS extending for a period greater than 12 hours (group A), the duration of CPB, interleukin-6 (IL-6), interleukin-8 (IL-8) and WBC count after aortic declamping were significantly longer and higher than those in patients with SIRS lasting less than 12 hours (group B). The duration of SIRS significantly correlated with the highest level of IL-6 (r= 0.724, p= 0.0038) and the duration of CPB (r= 0.626, p= 0.0201). These results suggest that the duration of CPB and cytokinemia, with high IL-6 levels, during this short time frame until just after cardiac surgery might play an important role in the development of the SIRS.(Ann Thorac Cardiovasc Surg 2003; 9: 365–70)