In order to evaluate cardiac involvement in bone marrow transplantation (BMT) we reviewed serial electrocardiograms, chest X-rays and cardiac pathology at autopsy in 45 consecutive adult patients undergoing BMT for hematologic malignancies at our institution. All patients were pretreated with cyclophosphamide (CY, 120 mg/kg) and total body irradiation (TBI, 10-12 Gy). A total of 15 patients developed electrocardiographic ST-segment or T-wave changes and/or arrhythmias post-BMT; four of them also suffered from congestive heart failure. The arrhythmias included frequent ventricular extrasystoles (two patients), paroxysmal atrial fibrillation (one patient), repeated supraventricular tachycardia (one patient) and QT-prolongation with ventricular tachyarrhythmias (one patient). Twelve patients showed an early drop of the total QRS voltage sum exceeding 15% of the pretransplant reading. In a subgroup of five patients the voltage drop was associated with ST-segment or T-wave changes; three of them developed congestive heart failure. At autopsy in 15 patients, the heart weight was on the average 113% of predicted. Myocardial edema, fibrosis and cellular hypertrophy were the most common microscopic findings. Two patients had marantic endocarditis of the aortic valve. Thus, in our experience clinically significant heart involvement affects 5-10% of patients undergoing BMT after pretreatment with CY and TBI. While cardiac complications are generally not a major problem of BMT, they may have serious consequences for individual patients and should therefore be carefully watched for in the treatment of BMT recipients.