An atrial septal defect is commonly thought of as a benign cardiac lesion especially in infancy. The haemodynamic consequences for the lungs, nonetheless, can be comparable to that produced by patency of the arterial duct. In a preterm boy, this lesion led to the development of broncho-pulmonary dysplasia, and dependence on a respiratory ventilator. He could be extubated shortly after surgical closure of the septal defect.