Background : The fenestrated Fontan operation has been beneficial to high–risk patients with univenricular hearts, resulting in improving survival. We performed the fenestration procedure in high-risk patients or patients had high CVP postoperatively after Fontan operation. We now perform a surgical modification for fenestrating using angled tube graft to facilitate coil embolization after extracardiac operation. Patients : Between Oct. 1998 and June 2003. 68 patients underwent fenestrated Fontan operation and 34 patients underwent cardiac catheterization after mean 31 months. We retrospectively reviewed 34 patients, divided into two groups according to patency state of fenestration after follow up. Results : A spontaneous closure was documented in 24 patients (70.6%). and a fenestration was patent in the other 9 patients. After test occlusion of fenestration, four patients underwent coil embolization and five patients with patent fenestration were considered inappropriate for closure. In spontaneous closure group, SaO2 increased from 84 to 94% (p<0.05) and cardiac index decreased from 4.7 to 3.7(p=0.006) but CVP or Rp did not changed significantly after operation. In patent group, none did not changed significantly after operation. There was no difference of preoperative datas significantly between two groups. In the spontaneous closure group, there were five Fontan failure patients; Late death (2), LCO syndrome (1) and protein-losing enteropathy (3, including 1 late death). Their preoperative datas were not significantly different from the other of same group. Conclusions : A spontaneous closure of these fenestration modification may occur easily in situations with good Fontan hemodynamics and in case of patent fenestration, a technique of fenestration allows simple, safe, and cost-effective closure available. However inappropriate closures of fenestration present in some patients but could not be predictable. Close observation after spontaneous closure in high-risk patients is needed.