The hemodynamic response to increasing left atrial pressure by volume loading was evaluated in 70 children during the first 24 hours after repair of congenital cardiac defects. The children were grouped into four diagnostic categories: atrial septal defect or pulmonary valve stenosis (n = 8), ventricular septal defect (n = 36), complete transposition after Mustard's operation (n = 13), and tetralogy of Fallot (n = 13). Within 2 hours of bypass, both cardiac index and left ventricular stroke work index were adequate and increased appropriately with volume loading in all four diagnostic groups. The atrial septal defect group demonstrated a similar response to volume loading 4 and 24 hours after bypass. However, the other three diagnostic groups had a higher filling pressure, lower cardiac index and stroke work index, and a depressed response to increasing preload 4 hours postoperatively, which indicated a deterioration in cardiac performance. The deterioration was maximal between 4 and 12 hours after bypass, and performance tended to recover 24 hours postoperatively. The transposition group had a more profound depression in cardiac performance than the other two groups. Within the ventricular septal defect group, smaller children (body surface area less than 0.36 m2) had a more profound depression in performance than larger children. These results demonstrate a significant alteration in cardiac performance during the first 24 hours after repair of congenital cardiac defects in children. These changes should be considered when postoperative management is being planned.