The purpose of this paper is to compare and define the postoperative hemodynamics, cardiac function and clinical status after the modified Fontan operation in patients with complex cardiac anomalies. Thirteen consecutive patients (6 with double-outlet right ventricle [DORV] [SLL : 4, SDL : 2], 5 with single ventricle [SV] [A-III : 3, B-III : 1, C-III : 1] and 2 with tricuspid atresia [TA] [Ib : 1, IIb : 1]) underwent the modified Fontan operation. These 13 patients ranging in age from 7 to 42 years of age (mean 18) were catheterized at 7 to 46 months (mean 8 months) postoperatively. Pressure tracings at rest demonstrated a dominant "a" wave in both the right atrium and the pulmonary artery suggesting a marked right atrial contraction after this operation. Left ventricular filling pressure was significantly decreased after the modified Fontan operation in DORV and SV. The ejection fraction and left ventricular end-diastolic volume index were unchanged or slightly increased after the modified Fontan operation. Cardiac index in 6 patients with DORV was significantly increased from rest to exercise by +48% (p less than 0.05) with a significant increase in the stroke volume (p less than 0.05) and with a slight increase in heart rate, but this index in 5 patients with SV and in 2 patients with TA was not significantly increased from rest to exercise. In the postoperative clinical status, 11 of 13 patients were in NYHA class I and 2 in class II at follow-up periods ranging from 4 to 75 months (mean 48 months). These results suggest that the modified Fontan operation can be of value and can provide excellent exercise tolerance for patients with complex cardiac lesions.