Some older patients develop symptoms of clinical heart failure after closure of an atrial septal defect (ASD). The present study tested the hypothesis that baseline hemodynamics and hemodynamic changes induced by transcatheter ASD closure are different between younger and older patients due to age-related differences in left ventricular (LV) diastolic dysfunction. Forty-three consecutive patients (27.7 ± 16.3 years of age, range 5-63, median 25) who underwent device closure for ASD were divided into younger (age ≤25, n = 24, 15.1 ± 1.2 years) and older (> 25 years, n = 19, 43.7 ± 2.2 years) groups. Echocardiographic evaluations were performed 1 day before and 2 days after ASD closure. Before ASD repair, early diastolic mitral annular velocity (e') on lateral, an index of ventricular relaxation, showed an age-related decrease. After closure, e' decreased by similar amount in both groups (p < 0.05). In addition, E/e', an index of LV filling pressure, was relatively unchanged in the younger group (from 5.4 to 5.9) but significantly increased (p < 0.05) in the older group (from 6.3 to 8.1) over similar increase of normalized LV diastolic dimension. In older patients, ASD closure resulted in further deterioration of baseline impairment in LV relaxation and the increased LV stiffness caused a more marked rise in LV filling pressure, compared to the younger group. Thus, ASD should be closed at a younger age before the development of age-related LV diastolic dysfunction.