Bronchial defects in neonates are known to occur very rarely as a complication of mechanical ventilation or intubation. This causes persistent air leakage that may form massive pneumomediastinum or pneumothorax, leading to cardiac tamponade or cardiorespiratory deterioration. Early diagnosis and treatment of bronchial defects are essential, as they can be accompanied by underlying severe lung parenchymal diseases, especially in preterm infants. We encountered an extremely low birth weight infant with an air cyst cavity in the posterior mediastinum that displaced the heart anteriorly, thereby causing cardiopulmonary deterioration. During exploratory-thoracotomy, after division of the air cyst wall (mediastinal pleura), we found a small bronchial defect in the posterior side of the right main bronchus. The patient had shown respiratory distress syndrome at birth, and she was managed by constant low positive pressure ventilation using a T-piece resuscitator after gentle intubation. As the peak inspiratory pressure was maintained low throughout and because intubation was successful at the first attempt without any difficulty, we think that the cause of the defect was not barotrauma or airway injury during intubation. The fact that the margin of the defect was very clear also suggested a congenital origin. To our knowledge, this is the first case of congenital bronchial defect in English literature.