In order to evaluate whether there is pulmonary vascular disease in patients with atrial septal defect (ASD), we used the pulmonary vascular input impedance to estimate the stiffness of the pulmonary vessels with before and after surgical intervention. Ten control subjects and 11 patients with ASD (9 operable and 2 inoperable) were examined. In preoperative patients the decreased total pulmonary resistance (Rin) and pulmonary vascular resistance appeared to open new parallel vascular channels with increased blood flow. Further, there were no significant differences in the pulmonary vascular input impedance spectrum and phase, and characteristic impedance among control subjects, preoperative and postoperative patients. Although distensibility of the pulmonary vascular wall in operable patients was similar to that in control subjects, an excessive elevation in Rin and input impedance modulus was observed in inoperable patients. The results demonstrate that the normal input impedance spectrum and phase in patients with ASD is predictive of a good prognosis after successful surgical correction.