Thoracoabdominal aortic aneyrusm (TAAA) repair results in ischemia-reperfusion injury and systemic inflammation. Nineteen patients who were undergoing TAAA repair were studied for the degree and mechanism of complement activation and its role in inflammation. Substantial complement activation was seen in these patients. Tumor necrosis factor-alpha (TNF-oc) interleukin-1 j and IL-8 increased significantly and correlated closely with complement activation. No increase in ILI,, TNFoc or IL-8, or in complement activation products was noted in a patient who was deficient in mannose-binding lectin. The authors concluded that MBL mediates complement activation during TAAA repair.