Acute liver dysfunction was analyzed in 15 patients who received a modified Fontan operation for single ventricle in nine (atrial isomerism, seven) and tricuspid or mitral atresia in six. Nine patients had elevation of serum glutamic-pyruvic transaminase levels above 1000 U/L during the first week. As an analysis of postoperative liver function during the first week, the highest values of serum glutamic-pyruvic transaminase and total bilirubin and the lowest prothrombin time were scored from 0 to 4 within each parameter, and totaled to give a liver dysfunction score. The liver dysfunction score was 0 to 2 (no or trivial injury) in five patients, 3 to 5 (mild) in two, and 6 to 11 (moderate or severe) in eight (53.3%). The group operated on for single ventricle had a higher incidence (67%) of a liver dysfunction score of 6 or higher than the other group (33%). A multivariate analysis for the prediction of the liver dysfunction score mainly from early postoperative hemodynamics showed the highest correlation with cardiac index, followed by urine output, systolic arterial pressure, and central venous pressure. One patient required plasmapheresis. Four died early (less than 1 month); three of these had a liver dysfunction score of 6 or higher. Those with scores of 6 or above had higher serum glutamic-pyruvic transaminase levels at 1 month after operation than those with scores less than 5. In three patients (single ventricle), hepatic venous oxygen saturation was monitored and showed a marked decrease to below 20% with subsequent acute liver dysfunction. These results indicate that acute liver dysfunction appears to occur in patients with complex lesions after a modified Fontan operation from possible hepatic hypoperfusion and that low cardiac output may be more crucial than high central venous pressure alone.