CASE PRESENTATION: A 51-year-old previously healthy male presented to the hospital with a four-week history of progressive dyspnea on exertion, orthopnea, and pleuritic chest pain, as well as, six months of 40-pound weight loss. On examination, he was found to be tachycardic with normal heart sounds, distended neck veins, and bilateral basal lung rales. Pulsus paradoxus was 24 mm Hg. Electrocardiogram showed sinus arrhythmia with tachycardia, diffuse low voltage QRS complexes and T-wave flattening in the inferior leads, without electrical alternans. His CBC showed leukocytosis with lymphocytic predominance. Other labs, including ESR, CRP, TSH, ANA, HIV, Rheumatoid Factor, syphilis and myasthenia gravis antibodies, were within normal limits. A Chest X-ray demonstrated an enlarged Cardiac silhouette and a large anterior mediastinal mass. Transthoracic echocardiogram showed a large pericardial effusion with tamponade physiology, as well as, a mass anterior to the right heart. An urgent pericardiocentesis was performed draining 640 ml of bloody fluid with the resolution of tamponade. Chest CT confirmed a large anterior mediastinal mass (16.3 x 19.5 x 20.6 cm) with central areas of necrosis and significant mass effect on the heart, trachea, and the bilateral main stem bronchi. Subsequently, the patient underwent mediastinoscopy with biopsy and flow cytometry demonstrating malignant Type B2 Thymoma. The patient was treated with neoadjuvant chemotherapy with cisplatin, doxorubicin, cyclophosphamide, and prednisone. He then underwent surgical debulking with plans for radiation therapy.