Chylothorax in adult occurs most commonly in the wake of cardiac and thoracic procedures. Injuries to the common thoracic duct in the thorax or its branches in the mediastinum, injuries to the thymus tissues, dissection of the superior vena cava or ascending aorta, dissection of the aortic arch, disruption of the accessory lymphatics in the left or right thorax, and increased pressure in the systemic vein exceeding that of the thoracic duct (usually in the superior vena cava thrombosis, Glenn Shunt, and hemi-Fontan) have been proposed as the possible causes of chylothorax after surgery for congenital heart disease. However, pulmonary hypertension is an exceedingly rare cause of chylothorax in adults. We present a case of chylothorax after atrial septal defect surgery in a 30-year-old female patient with pulmonary hypertension. The postoperative period was complicated by chylothorax, which was confirmed by the high lipid content of chylous effusion. The patient was treated conservatively with diet therapy, and the effusion was abolished completely after two weeks. No recurrence of chylothorax was detected at 3 months' follow-up.