A significant reduction (30%) in the amount of postoperative blood loss from 1.12 to 0.70 ml/kg/h was noted in pediatric patients subjected to cardiopulmonary bypass for congenital heart disease surgery when the activated whole blood clotting time was utilized to monitor anticoagulant therapy. However, no difference in the incidence (11 %) of postoperative hemorrhage was noted. Extensive preoperative laboratory hemo-stasis testing and routine immediate postoperative use of platelet transfusion and fresh frozen plasma (in cyanotic heart defects) failed to predict or correct those patients with significant postoperative hemorrhage.