CASE PRESENTATION: A 47 year-old male with HIV and Hepatitis B presented with 4 months of worsening productive cough and exertional dyspnea associated with subjective fevers, chills, and intermittent hemoptysis. Over that 4-month period, he had multiple admissions for pneumonia and one episode of chest pain with negative ECG and troponins at outside institution. He denied weight loss, sick contacts, and history of recent travel or tuberculosis exposure. On presentation, HR was 109bpm and exam revealed less than 1cm supraclavicular and inguinal lymphadenopathy with normal lung and cardiac exam. Chest X-ray revealed increased density along the right heart border. Chest CT scan showed a large 70x102mm soft tissue mediastinal mass invading the right atrium and right side of the left atrium, completely obstructing the superior right pulmonary vein, encasing the right main pulmonary artery, and compressing the right inferior pulmonary vein (Image 1). The mass was associated with supraclavicular, mediastinal, portal and retroperitoneal adenopathy. TTE showed vena caval inflow around the mass and elevated right ventricular pressures. Core biopsy of atrial mass revealed diffuse large B-cell lymphoma (DLBCL) with Ki-67 90%. Patient underwent 6 cycles of R-EPOCH and 4 cycles of prophylactic intrathecal methotrexate with complete resolution on follow up PET-CT (Image 2). His course was complicated by central nervous system lymphoma during 6-month follow up.