Background In a single ventricle, extracardiac Fontan operation has good early and midterm results such as low operative mortality and morbidity, but the optimal size of the conduit according to the patients��� growth is controversial. The optimal size of the conduit was investigated by reviewing the size of the conduit in patients who underwent extracardiac Fontan operation and measuring the inferior vena cava (IVC) area, the inferior vena cava-pulmonary artery (IVC-PA) length in people without the cardiac disease. Methods We reviewed the size of the conduit in 85 patients undergoing extracardiac Fontan operation between 1996 and 2002. The median age at extracardiac Fontan operation was 5.6짹5.0 years (range : 20 months to 25 years). The IVC size and the IVC-PA length were measured in 500 people who underwent chest computed tomography (CT) scan for noncardiac problems. Results The median size of conduit used in extracardiac Fontan operation was 19.4짹2.1 mm (range : 16 to 24 mm). The size of conduit increased according to the age, the body weight, the height and the body surface area of the patients. The IVC size and the IVC-PA length measured on chest CT scan of 500 people were correlated with the age, the body weight, the height and the body surface area. The IVC size and the IVC-PA length were increased by 20 years old. The IVC area and the IVC-PA length on chest CT scan in adults over 20 years old were 488짹127 mm2 and 78짹12 mm respectively. The actual size of the conduit used at extracardiac Fontan operation corresponds to about 78% of the measured diameter of the IVC in the adults. Conclusion We conclude the reoperation may be necessary in the patients undergoing relatively small sized conduit interposition when we compare it with the measured size of IVC in adults without the cardiac disease and consider the postoperative natural course of the conduits in extracardiac Fontan operation. And long-term follow-up studies are also needed for evaluating the optimal conduit size.