Spontaneous closure of intra-atrial baffle fenestrations occurred early postoperatively following a Fontan procedure in three patients (ages 0.8-2 yr) and were reopened using transvenous balloon dilation. Following this clinical experience with this complication we studied the effects of high pressure balloons of different sizes on fenestrations created in polytetrafluoroethylene (PTFE) patches. The intra-atrial baffle fenestration was found closed within hours after surgery in two patients and 32 days postoperatively in the other. Increasing edema, chest tube drainage, and renal dysfunction ensued rapidly. At catheterization the balloon dilation in two patients reduced the elevated right atrial pressure, edema, and pleural effusion and allowed an acceptable arterial saturation. In the other patient, with acidosis and a low cardiac output, transient cardiac arrest occurred following passage of the catheter through the fenestration and during later unsuccessful operation the fenestration, although open, was traversed by strands of clot. In the experimental study, a total of 45 fenestrations 4 mm in diameter were dilated with 5, 6, 7, 8, and 9 mm balloons. The fenestration areas remained increased significantly during the 14 days of observation in room air, with the larger balloons producing larger holes. Balloon dilation may be effective for spontaneous fenestration closure occurring in the early postoperative period after a fenestrated Fontan procedure. Our experimental data may be useful in selecting the balloon size in this setting.