Early surgery for forms of hypoplastic right heart syndrome may increase right ventricular size but could leave the patient with a residual right-to-left atrial shunt. Previous attempts to assess the capability of the right ventricle to accept systemic venous return have relied on angiographic estimates of tricuspid valve and right ventricular sizes. Since the minimum adequate sizes have not been established, we used a more physiologic technique of temporarily occluding the interatrial communication with a balloon-tipped catheter at cardiac catheterization in six consecutive patients. Five patients tolerated complete occlusion, although the tricuspid valve anulus diameter was less than the fifth percentile in all, and right ventricular volume was less than the fifth percentile in four. These five underwent surgical closure of an interatrial communication without evidence of postoperative systemic venous hypertension. Attempted occlusion in the sixth patient caused profound systemic venous hypoxia and surgical closure was not attempted. Temporary balloon occlusion may improve selection of patients for definitive operation.