We describe an unusual autopsy case of adult T-cell leukemia/lymphoma with massive cardiac involvement. The patient was admitted to the hospital with symptoms of fever and cervical lymph node enlargement, which improved following treatment with vincristine sulfate, cyclophosphamide (Endoxan), prednisolone sodium succinate, and doxorubicin hydrochloride (Adriamycin). He was then followed up in the outpatient clinic, but was readmitted to the hospital with palpitations and dyspnea. Cardiomegaly developed rapidly, and the patient died of congestive heart failure 3 months after readmission. The heart was massively enlarged at autopsy. The heart, including adhered surrounding tissue, weighed 1600 g. The myocardium was found to be replaced by massive infiltration of atypical lymphoid cells. Cardiac involvement by adult T-cell leukemia/lymphoma may result from extension or retrograde flow through cardiac lymphatics from the destroyed mediastinal lymphatic system.