We report the case of a 68-year-old man who presented with atrial flutter and was observed radiologically to have a large mass in the posterior mediastinum. During surgical removal, spontaneous recovery of sinus rhythm occurred. There was no late recurrence of arrhythmia. The diagnosis was mediastinal liposarcoma of mixed type (extremely rare). Supraventricular arrhythmia associated with mediastinal tumors is exceptional. Surgery appears to be the only potentially curative treatment for these tumors. In cases like ours presenting with arrhythmia, surgery is considered essential for control of the arrhythmia.