A 56-year-old man, suffering from rapidly worsening general fatigue, dyspnea on exercise and epigastralgia, was referred for evaluation and treatment of a cardiac tumor. Transthoracic echocardiography showed a 6-cm large mass occupying both the right ventricle and atrium. Gallium scintigraphy showed high uptake in the tumor site. Lymphoma was highly suggested. Urgent operation was performed and as much tumor and thrombus were removed as possible. The postoperative course was good, with cessation of right heart failure. Pathological examination suggested malignant lymphoma, diffuse large B-cell type. The patient was treated with rituximab, cyclophosphamide, Adriamycin, vincristine, and prednisone (CHOP-R) postoperatively and has survived for 2 years without signs of recurrence. (Circ J 2009; 73: 967 - 969)