The maze procedure was performed simultaneously with mitral valve replacement for atrial fibrillation and mitral stenosis in a 66-year-old female. On the first postoperative day, the cardiac status deteriorated suddenly due to a very rapid heartbeat which was later diagnosed as a 1:1 AV conduction of atrial flutter. The atrial flutter responded to an injection of verapamil and changed to a 2:1 conduction. Although atrial flutter had continued until the thirteenth postoperative day, the heart achieved sinus rhythm on that day. Since then it has continued without any antiarrhythmic drugs except digitalis. It was difficult to make a diagnosis of the rapid heartbeat on the 12-lead ECG because P waves could not be seen clearly on the ECG. Recording the electrical cardiac status by using the temporal atrial pacing leads, which were placed during the operation, was helpful in diagnosing the heartbeat as atrial flutter. The electrophysiological examination eight weeks after the operation revealed that the sinus node function was normal and atrial flutter or atrial fibrillation could not be induced. The patient is well and in sinus rhythm 12 months after the operation.