A 66-year-old woman admitted to our hospital with the complaints of abdominal distention, peripheral swelling, and diminished appetite. She had undergone mechanical mitral and aortic valve replacements 10 y ago. Chest radiography demonstrated a marked bulging of the right heart border misinterpreted as a mediastinal mass (Figure 1). Echocardiography showed a massively dilated right atrium secondary to severe tricuspid stenosis and regurgitation associated with normally functioning mechanical prostheses. On thorax computed tomography, transverse images gave the best overview of the giant right atrium which measured 145 × 156 × 180 mm with an estimated right atrial volume of 2,035 mL (Figure 2).