Between 1965 and 1995, 552 patients underwent closure of isolated secundum atrial septal defect, of whom 24 (4.3%) were infants with a mean age of 238.5 ± 13.8 days (range, 90 to 348 days). Mean weight was 6 ± 0.3 kg (range, 3.5 to 9 kg). Twenty-two had noted failure to thrive and 13 had chest infections. Direct closure was carried out in 20 and 4 underwent patch closure. There were 3 (12.5%) early deaths (intraoperatively, 5 hours, and 2 days postoperatively). Three patients had pulmonary hypertension with pulmonary arterial to systemic arterial pressure ratios of 0.74 to 0.83 preoperatively. Of 21 survivors, 13 were extubated within 24 hours and 8 within 72 hours of surgery. Mean hospital stay was 16.2 ± 8.2 days. There were 2 late deaths; one from pneumonia at 177 days postoperatively and another patient with a severe neuromuscular disorder who could not be extubated, died 328 days postoperatively. We concluded that some infants require early surgery and an atrial septal defect can be closed safely in infancy but the risk increases in patients with pulmonary vascular disease. The etiology of pulmonary vascular disease in such patients remains unclear.