Anticoagulation during CPB is accomplished by the administration of heparin, which is empirically dosed on the basis of patient weight. The degree of heparin-induced anticoagulation during cardiac surgery can be measured in the operating room by assessing heparin levels or by assessing heparin's anticoagulant effect with activated clotting times (ACTs). The most commonly used method is ACT monitoring. Unfortunately, the ACT is influenced by factors other than heparin, such as hemodilution and hypothermia. While not of major concern in adults and older children, the hemodilution accompanying CPB is extreme in neonates and young infants. Similarly, while most adult cardiac surgery is performed under normothermic or mild hypothermic conditions, the surgical procedures in infants are often conducted at extremely hypothermic temperatures. Therefore, ACT measurements during cardiac surgery in infants may not be reflective of the degree of heparin-induced anticoagulation. Indeed, heparin levels in young children are significantly lower than those found, maintained, and considered safe during CPB in adults while the ACT measurements in these children remain in the range felt to indicate adequate anticoagulation (6,7)