Fifty patients operated on for abdominal aortic aneurysms, ten acutely for ruptured aneurysms and 40 electively, were studied. The anaphylatoxins (C3a and C5a) and the activity of their inhibitor were determined preoperatively, regularly per-, and postoperatively. Increased anaphylatoxin activity was accompanied by altered airway pressure, central hemodynamics, and pulmonary, hepatic, and renal functions. During aortic clamping the anaphylatoxins increased and the activity of their inhibitor decreased. During the clamping period airway pressure, pulmonary artery pressure, and pulmonary vascular resistance increased. These respiratory and hemodynamic changes might partly be explained as biologic effects of anaphylatoxins. The plasma C3a increase correlated with the duration of aortic clamping. Patients with prolonged anaphylatoxin increase were prone to develop multisystem organ failure. Anaphylatoxins released during induced hypoperfusion could be one of the factors influencing respiratory and hemodynamic changes in patients operated on for aortic abdominal aneurysms.