Eleven cases of atrial septal defect, ostium secundum, with important symptomatology, under the age of one year are reported. The clinical picture does not differ essentially from what is found in older patients. Extracardiac associated anomalies are frequent. Pulmonary to systemic flow ratio varied between 1.3 and 3.5. The E.C.G. showed typical findings is most of the cases, but unexpected left ventricular enlargement was found in one case, and in another one with a Noonan syndrome, a left anterior hemiblock was present. Surgery was performed in six patients, between one and six years. In all, direct vision of the defect showed the large size of it. Pulmonary vascular resistances were normal. It is postulated that early vasodilatation of the pulmonary vascular bed could be the origin of the early onset of a left to right shunt in this patients without an increase in pulmonary vascular resistances.