We evaluated right and left atrial (RA and LA) volumes after modified Fontan operation in 6 patients, in 1 with mitral atresia and 5 with single ventricle. The age at operation was 10.2 +/- 2.5 (mean +/- SD) years. The interval between operation and study was 4.1 +/- 4.8 months. RA and LA maximum volume indexes (VImax) (ml/m2) were measured by angiography. One patient died 72 days after operation due to low output syndrome. In survived patients (n = 5), postoperative pulmonary arterial wedge pressure was 6 +/- 3 mmHg, right atrial pressure 13 +/- 5 mmHg, and end-diastolic pressure in the systemic ventricle 6 +/- 3 mmHg, and one who died had 18 mmHg, 25 mmHg, and 9 mmHg in each parameter, respectively. LAVImax in the survivors (53 +/- 18 ml/m2) were in the range of normal or above normal value. The nonsurvivor had extremely small LAVImax (13 ml/m2). RAVImax in the alive patients (25 +/- 11 ml/m2) was smaller than normal (data was not available in a non-survived patient). These results may indicate that a normal LA volume appears to be as important factor for the prognosis after modified Fontan operations with intraatrial routing for complex lesions regardless of the postoperative RA volume.