Acquired pulmonary stenosis is rare in adults and may be missed unless a high index of suspicion is present. Extrinsic pulmonic stenosis is even rarer and predominantly caused by external thoracic masses creating non-dynamic obstruction of the right ventricular outflow tract. A 20-year-old female was referred to our center with a cystic mass detected by transthoracic echocardiography and thoracic computed tomography, in the superoanterior mediastinum, 5.5x5.5x7 cm in size, causing main pulmonary trunk compression. Repeat transthoracic echocardiography demonstrated a mass causing compression of the main pulmonary artery immediately after the pulmonary valve. Continuous wave Doppler showed a peak systolic gradient of 65 mmHg and a mean gradient of 37 mmHg in the pulmonary artery at the site of compression. She underwent an open thoracotomy via a midline sternotomy. The mass was firmly attached to the pericardium. Its largest diameter was 15 cm; it surrounded the left phrenic nerve completely and invaded the outer wall of the pulmonary artery and aorta. The mass could only be partly dissected. The pathological diagnosis of the mass was stage IIa nodular sclerosing Hodgkin's lymphoma. The patient received postoperative chemotherapy and thoracal radiotherapy. She was in remission without any cardiac complaint.