Despite its known limitation in the diagnostic value, the electrocardiography is one of the most common and routine examinations in the management of the patients with cardiac problems. The clinical results of 291 patients who underwent isolated mitral valve replacement from October 1978 to June 1983 were already reported. Their electrocardiograms were studied to assess the value of electrocardiographic examination in following the patients after valve replacement. The patients were divided into 5 groups beforehand according to the types of valve lesion on the bases of preoperative diagnosis and operative findings: Groups I: stenosis, I1: stenosis-dominant mixed, II1: equally mixed, IV: insufficiency-dominant mixed, and V: insufficiency. Their preoperative cardiac rhythm was sinus in 39.2% and atrial fibrillation in 59.1% of the patients. Seventy-three [42.4%] of the patients with atrial fibrillation gained sinus rhythm after operation, occurring in 67 from the day of surgery, and 42 returned to atrial fibrillation while 37 kept sinus rhythm at the follow-up end [mean follow-up period, 13.4ulcorner1.4 months]. The P waves on the electrocardiograms of the preoperative sinus rhythm in 114 patients were normal in 5.3% and the findings of left atrial enlargement in 94.7% of the cases. They were normal in 42.1% and the findings of left atrial enlargement in 57.9% of the 140 patients with sinus rhythm at the follow-up. The preoperative major chamber enlargement was the right ventricle in Group I while it was the left ventricle in Group V, and it was in-between in Groups II-IV. The postoperative regression of the findings in ventricular enlargement was statistically significant only in Groups I and V. These results may suggest the importance of the serial electrocardiograms in following the patients with mitral valve replacement on the bases of outpatient. The electrocardiographic follow-up data were presented in patients with suspected or proved tissue valve failures.