We report a 26-year-old lady presenting with a huge mediastinal mass of diffuse large B-cell lymphoma. The patient had symptoms of shortness of breath, right-side chest pain and palpitations. Grade III/VI systolic murmur over the left upper sternal border was heard initially. This huge tumor, leaning outside the cardiac contour, was noted in the lateral circumference of the main pulmonary artery by echocardiography. The external compression of main pulmonary artery caused a turbulent flow and a peak systolic pressure gradient of 55.4 mmHg shown by continuous-wave Doppler examination. Two weeks after standard chemotherapy with CHOP (Cyclophosphamide, Adriamycin, Vincristine, Prednisolone), the peak systolic pressure gradient decreased to 19 mmHg and the systolic murmur over left upper sternal border disappeared. A rapid clinical improvement corresponding with shrinkage of the tumor was demonstrated.