A premature infant with respiratory distress syndrome developed a left chylothorax after evacuation of a left tension pneumothorax. The Argyle catheter noted in the posterosuperior mediastinum possibly traumatized the thoracic duct. Although the infant's chylothorax resolved with conservative management, he subsequently died of severe bronchopulmonary dysplasia. The potential complication of chylothorax from thoracostomy tubes can be prevented by avoiding penetration into the posterosuperior mediastinum.