Clinical characteristics of aging in congenital atrial septal defect (ASD) without significant right-to-left shunt (> 10%), were examined especially focusing on atrial arrhythmias such as atrial fibrillation (Af) and atrial premature contraction (PAC) in serial 48 patients, aged 42.6 +/- 2.3, ranging 16-67 years, Af was found in 12 (25%) out of 48 patients. Comparing patients with Af and those without Af, the average age (52.8 +/- 2.6 vs 38.6 +/- 2.8 years, p < 0.05), functional class of New York Heart Association (2.0 +/- 0.2 vs 1.6 +/- 0.1, p < 0.05) and left atrial dimension (41.4 +/- 1.1 vs 34.7 +/- 1.2 mm, p < 0.01) were significantly higher in the patients with Af than those without Af. Hemodynamic variables, i.e., mean right or left atrial pressure, Pp/Ps, Qp/Qs, left-to-right shunt, electrocardiographic R/S ratio in the lead V1 and tricuspid regurgitation had no relation either with aging or Af. Furthermore, daily PAC counts on Holter electrocardiograms significantly correlated with the age of the patients (r = 0.642, p < 0.05). These results suggest that the incidence of Af in congenital ASD is significantly related with aging/duration of ASD, but not with hemodynamic variables.