The complement system (CH50, C3, C4, C3 PA) was monitored during extra-corporeal circulation in 10 patients with ischemic-heart disease. Mean concentrations of CH50, C3, C4, and C3 PA significantly decreased as early as 5 minutes after initiation of the extra-corporeal circulation and decrement remain steady all over the surgical procedure. Transient neutropenia occurred early during the procedure. A significant hypoxemia was present only 24 hours after surgery. We suggest that a complement mediated leukostasis might occur with sequestration in the lungs and contribute to endothelial cell damage with pulmonary edema known as the "post-pump syndrome".