Four cases of atrial septal defect, "ostium secundum", associated with pulmonary hypertension and congestive heart failure in children under the age of two years are reported. A correct diagnosis could be made before surgery in only two of the case. In one case partial anomalous pulmonary venous connection was incorrectly diagnosed, while ventricular septal defect could not be excluded in the fourth case. The systolic pressure in the pulmonary artery was higher than 50 mm. Hg in all cases. One patient died after catheterization, and total surgical correction was performed in the other three cases, with good results. In all of them, a large atrial septal defect could be seen. There was no evidence of any other associated anomaly. After an average follow up of 4.5 years the three patients that where operated remain asymptomatic. The rarity of this entity, the difficulties in making a correct diagnosis and several pathogenetic aspects of this abnormality are discussed on the light of these and other cases previously reported in the literature.