A 50-year-old man was admitted to another hospital with epigastralgia. Malignant lymphoma was suspected because the patient had increased levels of serum LDH and an abnormal Ga scintigraphy finding in his chest. When he was transferred to our hospital, he underwent a right inguinal lymphadenopathy. The laboratory data showed increased levels of serum LDH and soluble IL-2 receptor, but there was no appearance of peripheral abnormal lymphocytes. His chest MRI indicated tumors in the right atrium (4 cm x 4 cm) and in the head of his left humerus. Those tumors were enhanced by Gd-DTPA. There were no other lymphadenopathies. Histopathological and immunohistochemical studies showed T-cell type lymphoma in the right inguinal lymph node. Furthermore, monoclonal rearrangement of HTLV-I proviral DNA was detected from the lymph node by Southern blot analysis. Taken together, we diagnosed the patient as having ATL (lymphoma type). His condition has improved well with systemic chemotherapy. We report a rare case of lymphoma type ATL with initial massive cardiac involvement, although ATL cells sometimes involve the heart at the end of the disease course.