A 27-year-old pregnant woman was admitted with supraventricular tachycardia (SVT) and symptoms of heart failure. Echocardiography revealed pulmonary hypertension due to a tumour infiltrating the left atrium and compressing the pulmonary veins. After delivery by Caesarean section, the paroxysmal SVT was controlled by amiodarone. Thoracic CT scan showed mediastinal masses compressing the pulmonary arteries and veins, and a preliminary diagnosis of Hodgkin's disease was later confirmed by mediastinoscopy and lymph node biopsy. Following two courses of chemotherapy the masses diminished. The lumen of the left atrium increased, pulmonary hypertension and SVTs receded.