To assess the long-term cardiac status after surgical closure of an atrial septal defect (ASD) at a young age, 104 of 135 children who consecutively underwent surgery (aged 0 to 14 years) at 1 institution between 1968 and 1980 participated in a follow-up study and underwent a complete cardiologic examination. Mean follow-up was 14.5 +/- 2.8 years. Most patients (87%) believed their health to be good or very good. At physical examination, all patients were in good health. Ninety-three patients (89%) were in sinus rhythm. Echocardiography showed that right ventricular dilatation was present in 27 patients (26%), 2 of whom had a residual ASD. Bicycle ergometry revealed that 88 patients (88%) had a normal exercise capacity. Both supraventricular and ventricular arrhythmias were observed in 67% of patients by 24-hour ambulatory electrocardiography, but only 3 (3%) had received antiarrhythmic medication, and 4 (4%) had needed a pacemaker. In the group of patients with right ventricular dilatation, the exercise capacity and prevalence of arrhythmias did not differ significantly from those in the group with a normal sized right ventricule. The outcome in patients with a secundum-type ASD was not different from that of those with a sinus venosus-type ASD. The finding of anatomic, functional or electrophysiologic abnormalities was not associated with a longer duration of follow-up.(ABSTRACT TRUNCATED AT 250 WORDS)