A 4-year-old child with hypoplastic left heart syndrome and a 6-year-old child with tricuspid atresia had both undergone staged reconstructive operations culminating in a Fontan operation. Peripheral edema, ascites, and hypoalbuminemia refractory to dietary manipulation and steroid therapy developed in both patients. After hemodynamic assessment, each child underwent surgical creation of a 4.8-mm fenestration in the previously placed baffle that separated the systemic venous pathway from the pulmonary venous atrium. Peripheral edema and ascites promptly resolved and serum protein levels normalized within 2 weeks after operation. Systemic arterial saturation is 86% in each child, and both children remain clinically well with no evidence of protein-losing enteropathy on normal diets and without specific medical therapy.