Children who undergo cardiopulmonary bypass (CPB) are proportionally more hemodiluted than adults who undergo CPB. Current methods of monitoring high-dose heparin sulfate anticoagulation are dependent on fibrinogen level. Because of the decreased fibrinogen levels in children, current methods of monitoring heparin anticoagulation overestimate their level of anticoagulation. Cildren and adults undergoing cardiac surgery who received CPB were anticoagulated in the standard fashion as directed by activated clotting time (ACT) results. Each subject had blood sampled at baseline; heparinization; start of CPB; CPB at 30, 60, and 90 minutes; and at termination of CPB. We assessed 2 subclinical markers of thrombosis, thrombin – antithrombin complexes and Prothrombin fragment F1.2. Fibrinogen was assessed as a marker of procoagulant reserve. Children had lower fibrinogen levels than adults throughout CPB (p<.05).The children had significantly higher thrombin – antithrombin complexes and Prothrombin fragment F1.2 than adults