Green dye measurements of cardiac index (in liters per minute per square meter) were compared in 32 infants and 58 children, 6 and 24 hours after normothermic cardiopulmonary bypass. Cardiac index was significantly lower in infants under one year of age, as compared to older infants and children. This parameter was also lower in patients with total anomalous pulmonary venous drainage, truncus arteriosus, and tetralogy of Fallot. Five infants who died had 6-hour cardiac index determinations of 2.0 L./min./sq. M. Only 2 of 27 survivors had cardiac indices below this level. Simultaneously recorded heart rate and central venous pressure did not correlate with cardiac index. Systolic blood pressure was significantly lower in infants in whom the cardiac index was less then 2.0 L./min./sq. M. (p less than 0.01). However, in individual situations, determination of systemic blood pressure lacked specificity in predicting outcome. Measurements of cardiac index are helpful in the management of infants and children following intracardiac repair by (1) assessing the hemodynamic status, (2) providing a useful indication of the cardiocirculatory effects of therapeutic interventions, and (3) predicting outcome.