In patients with an atrial septal defect and severe pulmonary hypertension, it is important to determine whether the latter is reversible before percutaneous or surgical closure. In addition to determining pulmonary resistance, one simple technique is to transiently occlude the septal defect using a balloon catheter and to evaluate the hemodynamic response. We defined a positive response as a > or = 25% reduction in mean pulmonary artery pressure during occlusion relative to the basal level, without a fall in systemic pressure or an increase in ventricular end-diastolic pressure. The study included five patients aged over 60 years with an atrial septal defect and severe pulmonary hypertension who were referred for percutaneous closure. In one patient, the test gave a negative result and closure of the atrial septal defect was not performed. In the remaining four, closure was indicated. In three patients, closure was performed percutaneously, while the fourth underwent surgery. The drop in pulmonary pressure observed during the test was maintained over the long term at a mean follow-up time of 22 months.