We experienced a surgery for an isolated right ventricular hypoplasia with atrial septal defect (ASD). The patient, 3 1/2-year-old girl, underwent direct closure of ASD. Preoperatively right ventricular end diastolic volume was 18 ml or 22 ml including trabecular portion by angiocardiographic examination. It was almost a half of normal. We performed intraoperative ASD occlusion test using a Foley 12 F balloon catheter before surgical closure of ASD. Postoperatively, mild right pleural effusion was found, but it disappeared soon with a use of diuretics. ASD occlusion test was safe and useful for estimating surgical closure of ASD. Postoperative cardiac catheterization revealed slight increase of right atrial pressure. It is necessary to follow up her condition carefully.