OBJECTIVE To report one case of a rare heart disease (Isolated Hypoplasia of the Right Ventricle) and discuss the methodology used to propose surgery with goal. Good follow up results. METHODS To report a case. METHODS One patient studied in the Cardiology Department of the Santa Marta Hospital, Lisbon, and submitted to surgery in the Cardiothoracic Surgery Department of the same Hospital. METHODS A 22 years old woman, who had cyanosis since birth, normal cardiac auscultatory findings, pulmonary oligohemia in the chest Roentgenogram and right atrial strain on the Electrocardiogram. METHODS She had an echocardiogram (M1 Mode and 2D) and catheterised, with pressure and oximetry measurements. During the latter procedure, the atrial septal defect was occluded for 20 minutes with a Fogarty catheter and pressure and oximetries were reevaluated. She was later operated. RESULTS The Echocardiogram showed right ventricular hypoplasia, with a normally inserted tricuspid valve and a dilated right atrium. The hemodynamic study confirmed the presence of an atrial septal defect with right to left shunt, normal pulmonary artery pressure, no gradients and giant "a" wave in the right auriculogram. The right ventriculography showed right ventricular inflow hypoplasia. The occlusion of the atrial septal defect with the balloon catheter decreased the peripheral insaturation, without a significant increase in the right ventricular pressure. With this information, surgery was proposed (closure of the atrial septal defect), with good results (4 years of follow-up). CONCLUSIONS The interruption of the shunt with a balloon allows us to see if the right ventricle can handle properly the volume load. With this information surgery can be suggested. It may be rather simple as in this case.